[easy] Fixed requires_grad preservation for nn.Module.load_state_dict(assign=True) (#121157)
Always preserve requires_grad of param in module. Documentation fixed in PR stacked above.
Also fix test case to test load a state_dict generated with `keep_vars=False` (the default)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121157
Approved by: https://github.com/albanD
