clarify hardtanh's definition
From the doc of https://pytorch.org/docs/stable/generated/torch.nn.Hardtanh.html#torch.nn.Hardtanh, the definition is very confused which seems to mean if ```x > max_val```, the output is ```1```, if ```x < min_val```, the output is ```-1```, I think we need to clarify the definition as this PR: if ```x > max_val```, the output is ```max_val```, if ```x < min_val```, the output is ```min_val```.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75789
Approved by: https://github.com/dagitses
