[maskedtensor] add basic tests and unary/binary/reduction tests from common_method_invocations (#82841)
Decided offline on the invariant that:
`masked_tensor` calls `MaskedTensor()`, which is analogous to `torch.tensor`
`as_masked_tensor` calls `MaskedTensor._from_values()`, which is analogous to `torch.as_tensor`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82841
Approved by: https://github.com/cpuhrsch, https://github.com/bhosmer
