Add sequence decoder integration tests

[jeffkinnison]

Adds integration tests for SequenceGeneratorDecoder in response to #3152.

[jeffkinnison]

@tgaddair The accuracy discrepancy I mentioned earlier caused the distributed test failures here. It looks like it's coming from the eval vs local comparison in tests.integration_tests.utils.train_with_backend.

[tgaddair]

@jeffkinnison that's what I figured. What this means is the distributed aggregation of the token_accuracy metric is not implemented correctly. We should probably fix that! Or if it's too much work, add the metric to the excluded list of metrics to compare in the test utils:

https://github.com/ludwig-ai/ludwig/blob/master/tests/integration_tests/utils.py#L908

[tgaddair]

Also, if you're going to cherry-pick this manually with the other fix, I will remove the release-0.7 label so it doesn't get auto-picked into the branch.

[github-actions]

Unit Test Results

         6 files  ±  0           6 suites  ±0   5h 42m 15s ⏱️ - 24m 25s
  4 010 tests +29    3 966 ✔️ +29    44 💤 ±0  0 ❌ ±0 
12 051 runs  +87  11 910 ✔️ +87  141 💤 ±0  0 ❌ ±0 

Results for commit f30bd50. ± Comparison against base commit 4f6d6ae.

♻️ This comment has been updated with latest results.