Parallelize tests on Circle CI.
Parallelize tests on Circle CI.

Set the number of CPUs manually based on the Circle CI resource class,
or else we're getting 36 CPUs, which is far too much (perhaps that's
the underlying hardware and not what Circle CI allocates to us).

Don't parallelize the custom tokenizers tests because they take less
than one second to run and parallelization actually makes them slower.

huggingface/pytorch-transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers