Add nvFuser support for aten.native_batch_norm_backward

[IvanYashchuk]

Replacing tensor.reshape(broadcast_mask) with unsqueezes makes the implementation of batch_norm_backward more friendly for PrimTorch+nvFuser.

[facebook-github-bot]

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[Chillee]

LGTM.

Note, there's a couple other uses of this pattern in this file, no need to fix them in this PR.

[IvanYashchuk]

@pytorchbot merge -g

[pytorchmergebot]

@pytorchbot successfully started a merge job. Check the current status here.
The merge job was triggered with the green (-g) flag. This means that your change will be merged once all checks on your PR have passed (ETA: 0-4 Hours). If this is not the intended behavior, feel free to use some of the other merge options in the wiki.
Please reach out to the PyTorch DevX Team with feedback or questions!

[pytorchmergebot]

Merge failed

Reason: The following mandatory check(s) failed (Rule superuser):

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[IvanYashchuk]

@pytorchbot merge -g

[pytorchmergebot]

@pytorchbot successfully started a merge job. Check the current status here.
The merge job was triggered with the green (-g) flag. This means that your change will be merged once all checks on your PR have passed (ETA: 0-4 Hours). If this is not the intended behavior, feel free to use some of the other merge options in the wiki.
Please reach out to the PyTorch DevX Team with feedback or questions!

[github-actions]

Hey @IvanYashchuk.
You've committed this PR, but it does not have both a 'release notes: ...' and 'topics: ...' label. Please add one of each to the PR. The 'release notes: ...' label should represent the part of PyTorch that this PR changes (fx, autograd, distributed, etc) and the 'topics: ...' label should represent the kind of PR it is (not user facing, new feature, bug fix, perf improvement, etc). The list of valid labels can be found here for the 'release notes: ...' and here for the 'topics: ...'.
For changes that are 'topic: not user facing' there is no need for a release notes label.