[docs] Remove `input` parameter from `Tensor.flatten` docs (#63180)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63180
**Summary**
This commit removes the `input` parameter from the signature for
`Tensor.flatten` shown in its documentation. This parameter is accepted
by `torch.flatten` but not `Tensor.flatten` (since the input is the
`Tensor` on which `flatten` is invoked).
**Test Plan**
Continuous integration.
**Fixes**
This commit fixes #57478.
Test Plan: Imported from OSS
Reviewed By: VitalyFedyunin
Differential Revision: D30293156
Pulled By: SplitInfinity
fbshipit-source-id: 4ad70d638af009fb6bdeb703433b306904d39a76
