Remove insert_observers pass (#24999)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24999
As described in previous PR, we are doing module level observer rather
than global observer now, so majority of code are deprecated. But we
still keeps some logic that is independent of this decision in the new
code.
Test Plan:
.
Imported from OSS
Differential Revision: D17001138
fbshipit-source-id: b456f80d587a61e368c626e7e8ac2a4f1282268b
