[MPS] Fix crash if nonzero is called concurrently (#108996)
Surrounds `stream->synchronize()` call with `dispatch_sync(stream->queue(), ^{});`, which is a noop for signle threaded program, but serializes calls to the synchronize across the threads using the same stream.
Prevent `[IOGPUMetalCommandBuffer validate]:215: failed assertion 'commit an already committed command buffer'` non-recoverable exception, which is triggered every time one is using PyCharm to inspect tensors on MPS device
Fixes https://github.com/pytorch/pytorch/issues/100285
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### <samp>🤖 Generated by Copilot at 1662ce2</samp>
> _Sing, O Muse, of the swift and skillful coders_
> _Who fixed the dreadful deadlock of the stream_
> _That crashed the mighty tensors of the MPS_
> _When they sought out the nonzero elements._
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108996
Approved by: https://github.com/kulinseth
