Replace torch.is_tensor usages with isinstance checks. (#38062)
Summary:
`is_tensor` doesn't really have a reason to exist anymore (other than
backwards compatibility) and is worse for typechecking with mypy (see
gh-32824). Given that it may not be obvious what the fix is once mypy
gives an error, make the change in a number of places at once, and add
a note on this to the `is_tensor` docstring.
Recommending an isinstance check instead has been done for quite a
while, e.g. https://github.com/pytorch/pytorch/pull/7769#discussion_r190458971
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38062
Differential Revision: D21470963
Pulled By: ezyang
fbshipit-source-id: 98dd60d32ca0650abd2de21910b541d32b0eea41
