[WIP] filter API

[eric-haibin-lin]

Description

(Brief description on what this PR is about)

Checklist

Essentials

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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request head (filter@7c6c713). Click here to learn what that means.
The diff coverage is n/a.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request head (filter@5a000eb). Click here to learn what that means.
The diff coverage is n/a.

[leezu]

Thanks @eric-haibin-lin. I added a small change to explicitly test against None instead of taking the boolean value only: 5a000eb

[eric-haibin-lin]

moved to apache/mxnet#16078