catch get_dataset_infos error for community datasets

[VictorSanh]

Some community datasets don't have dataset_infos.json.
This will return an error in the helicopter view

Also drastically reduced the number of workers in multiprocessing (which I think was causing some issues).

[stephenbach]

@VictorSanh @tianjianjiang Does this PR also address the issue in #411? Is there anything from #411 that should be included here?

[tianjianjiang]

@VictorSanh @tianjianjiang Does this PR also address the issue in #411? Is there anything from #411 that should be included here?

@stephenbach +CC @VictorSanh I will have to test it again to be sure. However, last time I tested it, cpu_count() wouldn't fix #411, and cpu_count() was much slower than len(en_datasets), probably because it was I/O bound.

[VictorSanh]

i removed the temptative fix for pool. let's have it in another PR.
I am merging things now cause people are asking for this fix