[FEATURE] MXIndexedRecordIO: avoid re-build index

[huww98]

Description

When resetting, avoid re-build index for read-only instances to save time and memory (in case of forked, memory pages can be shared)

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

• avoid reset index

Comments

This can save ~30G memory for us when running with 16 forked workers.

[mxnet-bot]

Hey @huww98 , 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]

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

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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.

[ptrendx]

LGTM