wrap models with DistributionalQModel when running DQN

[AmeerHajAli]

Why are these changes needed?

Prevents circular dependency of custom models when running DQN - #6091.
I also added a test that checks internal custom models for DQN and makes sure it does not accidentally break for other policies. PR after code review of PR #6254.

Related issue number

Closes #6091

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://ray.readthedocs.io/en/latest/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failure rates at https://ray-travis-tracker.herokuapp.com/.

[AmplabJenkins]

Can one of the admins verify this patch?

[AmplabJenkins]

Test FAILed.
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https://amplab.cs.berkeley.edu/jenkins//job/Ray-PRB/18833/
Test FAILed.

[AmeerHajAli]

jenkins, retest this please

[AmplabJenkins]

Test FAILed.
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Test FAILed.

[AmeerHajAli]

I reran the same test that failed (test_checkpoint_restore.py) locally and it passes. :/

[ericl]

jenkins retest this please

[ericl]

jenkins retest this please

[AmplabJenkins]

Test FAILed.
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Test FAILed.

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[ericl]

Merged, thanks!