CSC 485H/2501H: Computational linguistics, Fall 2024 - Assignment 2

Due date: 17:00 on Thursday, November 7, 2024.

General Instructions

• Late assignments will not be accepted without a valid medical certificate or other documentation of an emergency.
• For CSC485 students, this assignment is worth 33% of your final grade. For CSC2501 students, it is worth 25% of your final grade.
• Read the whole assignment carefully.
• Type the written parts of your submission in no less than 12pt font.
• Your work must be your own. Do not work with anyone else on any of the problems. If you need assistance, contact the instructor or TA.
• Any clarifications to the problems will be posted on the Discourse forum for the class. Check the page regularly.
• The starter code directory for this assignment is distributed via MarkUs. Refer to code files in that directory.
• When implementing code, read the docstrings as some provide important instructions, implementation details, or hints.
• Fill in your name, student number, and UTORid on the relevant lines at the top of each file you submit.

0. Warming up with WordNet and NLTK (4 marks)

(a) Deepest function (1 mark)

• Implement the deepest function in q0.py to find the synset in WordNet with the largest maximum depth and report both the synset and its depth on each of its paths to a root hyperonym. Hint: use wn.all_synsets and synset.max_depth methods.

(b) Superdefn function (2 marks)

• Implement the superdefn function in q0.py that takes a synset s and returns a list consisting of all of the tokens in the definitions of s, its hyperonyms, and its hyponyms. Use word_tokenize as shown in chapter 3 of the NLTK book.

(c) Stop_tokenize function (1 mark)

• Implement the stop_tokenize function in q0.py that takes a string, tokenizes it using word_tokenize, removes any tokens that occur in NLTK’s list of English stop words and also removes any tokens that consist entirely of punctuation characters. Use Python’s punctuation characters from the string module. Maintain the original case in the return value.

1. The Lesk algorithm & word2vec (28 marks)

(a) Mfs function (1 mark)

• Implement the mfs function that returns the most frequent sense for a given word in a sentence. Note that wordnet.synsets() orders its synsets by decreasing frequency.

(b) Lesk function (6 marks)

• In the lesk function in q1.py, implement the simplified Lesk algorithm as specified in Algorithm 1, including Overlap. Overlap(signature, context) returns the cardinality of the intersection of the bags signature and context. Use stop_tokenize function to tokenize the examples and definitions.

(c) Lesk_ext function (3 marks)

• In the lesk_ext function in q1.py, implement a version of Algorithm 1 where the signature also includes the words in the definition and examples of sense’s hyponyms, holonyms, and meronyms. Use stop_tokenize as before.

(d) Justification for lesk_ext (2 marks)

• Explain why the extension in lesk_ext is helpful. Consider the likely sizes of the overlaps.

(e) Lesk_cos function (4 marks)

• In the lesk_cos function in q1.py, implement a variant of lesk_ext that uses CosSim instead of Overlap. Modify signature and context to be vector-valued and construct the vectors as described. Use stop_tokenize to get the tokens for the signature.

(f) Lesk_cos_oneside function (2 marks)

• In the lesk_cos_oneside function in q1.py, implement a variant of lesk_cos that, when constructing the vectors for the signature and context, does not include words that occur only in the signature. Use stop_tokenize to get the tokens for the signature.

(g) Comparison of lesk_cos_oneside and lesk_cos (3 marks)

• Compare how well lesk_cos_oneside performs compared to lesk_cos. Justify your answer with examples.

(h) CosSim with binary values (1 mark)

• If we use CosSim for vectors with binary values (representing sets), how is it related to the set intersection? (No implementation required.)

(i) Lesk_w2v function (4 marks)

• In the lesk_w2v function in q1.py, implement a variant of lex_cos where the vectors for the signature and context are constructed by taking the mean of the word2vec vectors for the words in the signature and sentence, respectively. Treat the signature and context as sets rather than multisets. Use stop_tokenize to get the tokens for the signature.

(j) Lowercasing tokens (2 marks)

• Alter your code so that all tokens are lowercased before they are used for any of the comparisons, vector lookups, etc. Analyze how this alters the different methods’ performance and explain why. Do not submit this lowercased version.

2. Word sense disambiguation with BERT (22 marks)

(a) Context necessity (4 marks)

• Is context really necessary? Give an example of a sentence where word order–invariant methods such as those implemented for Q1 will never be able to completely disambiguate. Explain the more general pattern and why these methods cannot provide the correct sense for each ambiguous word.

(b) Gather_sense_vectors function (10 marks)

• Implement gather_sense_vectors in q2.py to assign sense vectors as described.

(c) Sorting corpus for gather_sense_vectors (2 marks)

• In the docstring for gather_sense_vectors, explain why sorting the corpus by length before batching is much faster than leaving it as-is. Hint: think about padding.

(d) Bert_1nn function (4 marks)

• Implement bert_1nn in q2.py to predict the sense for a word in a sentence given sense vectors produced by gather_sense_vectors. Keep in mind the note in the docstring about loop usage.

(e) Issues with arbitrary sentences (2 marks)

• Think of at least one other issue that would come up when attempting to use the code for this assignment to disambiguate arbitrary sentences. Consider either the Lesk variants from Q1 or the BERT-based method here (or both).

3. Understanding transformers through causal tracing (16 marks)

(a) Get_forward_hooks function (3 marks)

• Implement get_forward_hooks.

(b) Causal_trace_analysis function (5 marks)

• Implement causal_trace_analysis to compute the impact of states, MLP and attention.

(c) Causal tracing result report (1 mark)

• Report your generated causal tracing result plots for the prompt “The Eiffel Tower is located in the city of” with the output “Paris” in your report.

(d) Model size impact on causal tracing (3 marks)

• Experiment with different sizes of GPT-2 models (e.g., small, medium, large, and XL) to examine how model size impacts causal tracing patterns. Address the following in your report:
  • At what model size do you observe that the causal tracing pattern no longer appears?
  • Discuss potential reasons for how and why this change in causal tracing patterns occurs as the model size increases or decreases.

(e) Prompt types for similar causal tracing patterns (2 marks)

• Using GPT-2 XL, experiment with various prompts to identify prompt types that result in a causal tracing pattern similar to the one illustrated in Figure 2. Document your findings with examples and discuss what characteristics of the prompts might contribute to this similarity.

(f) Absent or diminished causal tracing patterns (2 marks)

• For GPT-2 XL, explore different prompts and tasks to find cases where the causal tracing pattern is absent or significantly diminished. Describe the prompt/task and hypothesize why the pattern does not emerge. Discuss any trends or patterns you identified and reflect on the broader implications of how language models process, store and generate factual information obtained from pretraining.

What to submit

• Submit electronically via MarkUs.
• Submit a total of five required files:
  • a2written.pdf: a PDF document containing answers to questions 0a, 1d, 1f, 1h, 2a, and 2d. Also include a typed copy of the Student Conduct declaration and sign it by typing your name.
  • q0.py: the entire file with your implementations filled in.
  • q1.py: the entire file with your implementations filled in. Do not include the alterations for question 1h.
  • q2.py: the entire file with your implementations filled in.
  • q3.py: the entire file with your implementations filled in.

CSC485 A2

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