[FEATURE][OP] Support FInplaceIdentity property for legacy operators

[shuo-ouyang]

Description

We notice that only nnvm operators support FInplaceIdentity property but legacy operators do not, which leads to InplaceOption failing when the storage reference count is large than 1. We add FInplaceIdentity support for legacy operators in this PR.

Checklist

Essentials

• [x] PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• [x] Changes are complete (i.e. I finished coding on this PR)
• [ ] All changes have test coverage
• [x] Code is well-documented

Changes

• [ ] Feature1, tests, (and when applicable, API doc)
• [ ] Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

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