Fix type hints on nn.attention.sdpa_kernel (#119140)
Fixes #119133
Altered type hint and assert to include SDPBackend; disallowed None in assert.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119140
Approved by: https://github.com/mikaylagawarecki, https://github.com/cpuhrsch, https://github.com/drisspg
