[SDPA] update type hint for scaled_dot_product_attention and documentation (#94008)
# Summary
- Adds type hinting support for SDPA
- Updates the documentation adding warnings and notes on the context manager
- Adds scaled_dot_product_attention to the non-linear activation function section of nn.functional docs
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94008
Approved by: https://github.com/cpuhrsch
