Support torchtext on a single GPU

[elkotito]

What does this PR do?

Allows using torchtext on a single GPU e.g. via torch.text.BucketIterator object which in turn generate torchtext.data.Batch objects. It seems like it works only on a single GPU due to #2350.

Fixes #1245

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[awaelchli]

@mateuszpieniak I wonder why torchtext.Batch doesn't have a .to() method. Wouldn't that be a logical choice? It seems so obvious. It would make the support much easier.

[Borda]

LGTM

[williamFalcon]

awesome! let's finalize the changes requested so we can merge :)
Great PR

[elkotito]

1. I need some help with the docs. I updated the docs with Batch class info (code/hooks.py), but for some reason, it is grayed out. Is there anything else I have to do to make it work?

    The data types listed below (and any arbitrary nesting of them) are supported out of the box:

    - :class:`torch.Tensor` or anything that implements `.to(...)`
    - :class:`list`
    - :class:`dict`
    - :class:`tuple`
    - :class:`torchtext.data.batch.Batch`

2. Is the requirements/extra.txt correct to put torchtext dependency?

[Borda]

Torchtext as mandatory dependency? When we refused torchvision.... @williamFalcon

[awaelchli]

agree, should go to extras. I was not fast enough to properly review :(

[Borda]

fixing in #2395

[williamFalcon]

sorry, didn't catch that. i thought it was in extras