Fix linalg_eigvals invalid use of composite dispatch key (#121142)
`linalg_eigvals_out` calls into a dispatch stub, so only supports CPU and CUDA
strided tensors but incorrectly claimed to be a composite op. `linalg_eigvals`
also shouldn't defer to the out variant inside a `CompositeImplicitAutograd` op
as not all types support out variants. Instead, I add a new helper
`_linalg_eigvals` which does the same thing in a non-composite operator.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121142
Approved by: https://github.com/lezcano
