[Bug Fixed] Add quote for train_cmd in metaseq/launcher/slurm.py

[wkcn]

Hi there, thanks for your great work!
I found that there is no quote for train_cmd in metaseq/launcher/slurm.py.
It leads to an issue that dry-run prints a command without quote, namely --adam-betas (0.9, 0.95), which could not run directly. The correct command should be --adam-betas '(0.9, 0.95)'.

Patch Description
Add quote for train_cmd in the function gen_train_command of metaseq/launcher/slurm.py.
The command --adam-betas (0.9, 0.95) will be fixed to --adam-betas '(0.9, 0.95)' in this PR : )

Testing steps
Run the following command to print the dry-run message.

opt-baselines \
  -n 1 -g 2 \
  -p test_v0 \
  --model-size 125m \
  --azure \
  --data ./dataset \
  --checkpoints-dir "./outputs" \
  --local \
  --dry-run

[suchenzang]

This commit is being reverted in #69. Opened an issue to add an integration test to catch these errors before merging: #68