[FEATURE] Add feature of attach_grad to intermediate variables in HybridizedBlock.

[KexinFeng]

Description

The PR adds the support for fetching the gradients of intermediate variables in a gluon HybridizedBlock. This applies when block.hybridize() is on; otherwise, calling the block would be a simpler imperative computation where the original attach_grad() can be directly applied, as implemented in PR#20500.

The motivation of this feature comes from this issue#11865.

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

• This feature is built on top of PR#20500, where the modification is mainly in invoke of CachedOp computation.

[mxnet-bot]

Hey @KexinFeng , Thanks for submitting the PR
All tests are already queued to run once. If tests fail, you can trigger one or more tests again with the following commands:

• To trigger all jobs: @mxnet-bot run ci [all]
• To trigger specific jobs: @mxnet-bot run ci [job1, job2]

---

CI supported jobs: [unix-cpu, centos-cpu, clang, windows-gpu, website, centos-gpu, windows-cpu, miscellaneous, edge, unix-gpu, sanity]

---

Note:
Only following 3 categories can trigger CI :PR Author, MXNet Committer, Jenkins Admin.
All CI tests must pass before the PR can be merged.