[MPS] Better error message for `slow_conv2d_forward` (#86303)
Error `Could not run 'aten::_slow_conv2d_forward' with arguments from the 'MPS' backend.` is very misleading as usually this method is only invoked if input is on CPU but weights are on MPS device.
Raise a more user friendly error in this case
Add test to `test_invalid_conv2d` to check for those conditions.
Fixes https://github.com/pytorch/pytorch/issues/77931
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86303
Approved by: https://github.com/kulinseth
