[Feature] Enable ninja by default

[IvDmNe]

What is the feature?

Currently (in 2.2.0) building wheels invokes ninja if it is installed to parallelize jobs. When building wheels with mim install mmcv (for example, for different python version) only one core is used. And it is not obvious that additionally installing ninja can enable multiprocessing here, leading to very long build time.

What were the reasons not to include ninja as necessary dependency and default option when building wheels?

If it is not possible to enable ninja by default, I suggest to add warning in case ninja is not installed to notify users who want to install mmcv as third-party dependency (I think... almost all users do it this way) about ninja option.

Any other context?

No response