Add float cast tensor op

[laggui]

Checklist

• Confirmed that run-checks all script has been executed.
• Made sure the book is up to date with changes in this PR.

Changes

Add tensor.cast(dtype) op for floating point casting to different precision types.

Currently implemented for candle and tch. Had to remove the element type generic on both tensor primitives for this change.

Will be implemented in a follow-up PR for: burn-ndarray, burn-jit, burn-fusion and burn-router.

Important: handling different floating point precision types in operations between multiple tensors might require automatic type promotion based on a selected strategy (e.g., cast to the higher precision type like torch). This is not in the current scope, instead backends can panic with a dtype mismatch.

Testing

Added a unit test.

[codecov]

Codecov Report

Attention: Patch coverage is 38.93443% with 298 lines in your changes missing coverage. Please review.

Project coverage is 82.85%. Comparing base (a4567db) to head (1018228).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/burn-tch/src/ops/int_tensor.rs	18.07%	68 Missing ⚠️
crates/burn-tch/src/ops/module.rs	3.22%	60 Missing ⚠️
crates/burn-tch/src/ops/tensor.rs	54.23%	54 Missing ⚠️
crates/burn-tch/src/ops/base.rs	52.05%	35 Missing ⚠️
crates/burn-tch/src/ops/bool_tensor.rs	30.76%	18 Missing ⚠️
crates/burn-tensor/src/tests/ops/cast.rs	0.00%	9 Missing ⚠️
crates/burn-tensor/src/tensor/element/base.rs	0.00%	8 Missing ⚠️
crates/burn-candle/src/ops/base.rs	83.33%	7 Missing ⚠️
crates/burn-candle/src/ops/tensor.rs	57.14%	6 Missing ⚠️
crates/burn-tensor/src/tensor/api/float.rs	0.00%	6 Missing ⚠️
... and 7 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2483      +/-   ##
==========================================
- Coverage   82.97%   82.85%   -0.13%     
==========================================
  Files         812      814       +2     
  Lines      105212   105351     +139     
==========================================
- Hits        87304    87288      -16     
- Misses      17908    18063     +155     

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