[FEA][webdatamodule]: support webdataset invocable (#501)

## Summary
The webdataset invocables are member functions of the webdataset or
webloader class that return the the same instance their are invoked
from. Previously webdatamodule does its own computation on the epoch
length but now can rely on the user input directly using
`webdataset/webloader.with_epoch()` due to this new additional feature


## Details
_Describe your changes. You can be more detailed and descriptive here.
If it is a code change, Be sure to answer:_
  - _What is changing?_

1. Add user input webdataset invocables and apply them to the
webdataset/webloader objects upon construction.
2. Remove the computation related to epoch length and update the tests
correspondingly.
3. Update the doc to reflect the change

  - _What is the new or fixed functionality?_

(see the summary above)

  - _Why or when would someone want to use these changes?_

These invocables are parts of the webdataset usage and we have been
using them in diffdock to set up the epoch length and other properties
of the dataset and dataloader objects.

  - _How can someone use these changes?_

(see the updated README.md regarding the `invoke_wds` and `invoke_wld`
arguments on webdatamodule)


## Usage
_How does a user interact with the changed code?_
(see the updated README.md regarding the `invoke_wds` and `invoke_wld`
arguments on webdatamodule)

## Testing

Tests for these changes can be run via:
```shell
pytest -v subpackages/bionemo-webdatamodule/tests
```

**Most of the changes** to files with extensions `*.py`, `*.yaml`,
`*.yml`, `Dockerfile*` or `requirements.txt` **DO REQUIRE both `pytest-`
and `jet-` CI stages**.

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before asking for review?
- [x] Did you make sure your changes have tests? Did you test your
changes locally?
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