consistent nn. and nn.functional: part 5 docs
Continuing https://github.com/huggingface/transformers/pull/12124 this PR takes care of `docs` - had to do a bit of extra filtering to not break the images:

```
# deal with torch.nn
perl -pi -e 's|^(\s*)import torch\n|$1from torch import nn\n$1import torch\n|' `grep -Ilr torch.nn docs`
find docs -type f -regextype posix-egrep -regex '.*(md|rst)$' -exec perl -X -pi -e 's{(?<!(from |import |[`#/]))torch\.nn\.}{nn.}g' {} \;
find docs -type f -regextype posix-egrep -regex '.*(md|rst)$' -exec perl -pi -e 's|import torch\.nn as nn|from torch import nn|g' {} \;

# deal with F
find docs -type f -regextype posix-egrep -regex '.*(md|rst)$' -exec perl -pi -e 's|from torch.nn import functional as F|from torch import nn|g' {} \;
find docs -type f -regextype posix-egrep -regex '.*(md|rst)$' -exec perl -pi -e 's|import torch.nn.functional as F|from torch import nn|g' {} \;
find docs -type f -regextype posix-egrep -regex '.*(md|rst)$' -exec perl -pi -e 's|(?<!\w)F\.|nn.functional.|g' {} \;


make fixup
```
and one manual tweak. docs are hard to automate rewrites and there is no validation, so had to carefully check each diff.


@sgugger 