dispatch API for checking computed table, use it in prim decomps (#82358)
Fixes https://github.com/pytorch/pytorch/issues/82331
Expose a `torch._C._dispatch_has_computed_kernel_for_dispatch_key` to check if an operator has a kernel registered to the given dispatch key in the **computed table**.
Use it in the prim registration logic, making it more accurate and robust (so that it e.g. picks up `CompositeExplicitAutograd` kernels.
It looks like before this change we'd register 134 prim ops to the meta key, and after we only register 62. So that's 72 ops that now use an existing C++ decomp to get meta working, instead of going directly through the prim decomp.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82358
Approved by: https://github.com/ezyang
