[BE]: Add type alias typing annotation to prims_common

[Skylion007]

Explicitly mark unions assignments as type aliases to make it easier for static type checkers.

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit d49db16 with merge base af83141 ():

UNSTABLE - The following job failed but was likely due to flakiness present on trunk and has been marked as unstable:

• pull / linux-focal-py3_8-clang9-xla / test (xla, 1, 1, linux.12xlarge, unstable) (gh)
  Process completed with exit code 128.

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[ezyang]

@pytorchbot merge

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