Add ability to assign classes for ImageFolderDataset

[dwSun]

there should be a convenient way to assign classes and the order of them.

[mxnet-bot]

Hey @dwSun , 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:

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[dwSun]

@mxnet-bot run ci [all]

[mxnet-bot]

Jenkins CI successfully triggered : [edge, centos-cpu, unix-gpu, website, centos-gpu, windows-gpu, clang, unix-cpu, windows-cpu, sanity, miscellaneous]

[szha]

@mxnet-bot run ci [all]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-cpu, edge, windows-cpu, sanity, website, unix-gpu, windows-gpu, centos-cpu, miscellaneous, clang, centos-gpu]