Further improvements to EmbeddingBagCollection documentation and related classes

[rodrigodesalvobraz]

Summary: Continue improvements to EmbeddingBagCollection:

• replace LaTeX-like notation _{f,i} by Python-like notation [f][i]
• Improves spatial formatting of example for readability and rendering in browser.

Futhermore, improve documentation of related classes:

• EmbeddingCollection had documentation similar to the previous version of EmbeddingBagCollection documentation, so it is now updated to the style of the new documentation, for consistency as well as greater precision and clarity.

• fbcode/torchrec/quant/embedding_modules.py redefines EmbeddingBagCollection in a distinct package, and simply duplicated the main implementation of the original class, with a few differentiating comments. Instead of merely copying-and-pasting the new documentation to this class, we change the documentation preserve the differentiating comments and merely refer to the original class documentation. This prevents duplication and possible future inconsistencies, and mirrors what other redefinitions of this class do.

Differential Revision: D66281917

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D66281917

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D66281917

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D66281917