[spmd] DistributedTensor doesn't implement is_pinned

[kumpera]

As part of FSDP+TP integration we use construct Tensors using TensorInfo.from_tensor which calls is_pinned:

I'm getting the following error with it:

  File "/data/home/kumpera/repos/PiPPy/spmd/spmd/tensor/dispatch.py", line 174, in operator_dispatch
    raise RuntimeError(
RuntimeError: Operator aten.is_pinned.default does not have a DistributedTensor rule registered.

As an work-around, I'm commenting the relevant code.