Update padding_idx docs for EmbeddingBag to better match Embedding's (#56065)
Summary:
Match updated `Embedding` docs from https://github.com/pytorch/pytorch/pull/54026 as closely as possible. Additionally, update the C++ side `Embedding` docs, since those were missed in the previous PR.
There are 6 (!) places for docs:
1. Python module form in `sparse.py` - includes an additional line about newly constructed `Embedding`s / `EmbeddingBag`s
2. Python `from_pretrained()` in `sparse.py` (refers back to module docs)
3. Python functional form in `functional.py`
4. C++ module options - includes an additional line about newly constructed `Embedding`s / `EmbeddingBag`s
5. C++ `from_pretrained()` options
6. C++ functional options
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56065
Reviewed By: malfet
Differential Revision: D27908383
Pulled By: jbschlosser
fbshipit-source-id: c5891fed1c9d33b4b8cd63500a14c1a77d92cc78
