Add support for single tensor in `inputs` argument for backward (#53827)
Summary:
Also updates the doc such that the language matches the type. For example, previously the `tensors` argument is specified as `(sequence of tensor)`, but has type annotation of `_TensorOrTensors`. Now its correctly updated to be `Sequence[Tensor] or Tensor`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53827
Reviewed By: albanD
Differential Revision: D26997541
Pulled By: soulitzer
fbshipit-source-id: e1e609a4e9525139d0fe96f6157175481c90d6f8
