[export] fix canonicalization for input mutations (#119533)
The comparison was off: user_input_mutation and buffer_mutation had the same numeric value, which led the comparison to move to the next element of the tuple and try to compare `None` to `spec.buffer_mutation.buffer_name`, which doesn't work. So make them different numbers.
Differential Revision: [D53601300](https://our.internmc.facebook.com/intern/diff/D53601300/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119533
Approved by: https://github.com/zhxchen17
