Check file size within LocalUploader

[XiaohanZhangCMU]

Description of changes:

LocalUploader can be used by fuse-mount file system which may be not as reliable. We have seen upload finishing but the shard files are missing, however no errors were thrown. Thus adding a check to assert local file size matches the remote file size.

Issue #, if available:

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[srowen]

I think it's a good check. From our ongoing side conversation, I think this probably isn't the issue (the copy never succeeds). But this never hurts as a double-check I think.