[fx] fix merge_matmul tests making invalid torch.split calls
The merge_matmul tests currently call `torch.split` with a list of
`torch.fx.Attribute` objects which won't work once `torch.split` is moved into
C++ because the argument parser actually checks the list contains integers
before calling `__torch_function__`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75019
Approved by: https://github.com/jamesr66a, https://github.com/jfix71
