Add processing of `PoolGenerator` class for `pool` parameter in `Auto.__init__`

[Mr-Geekman]

🚀 Feature Request

Currently, the type hint for pool is Union[Pool, List[BasePipeline]]. It is strange that PoolGenerator isn't possible.

Proposal

Make type hint: Union[Pool, PoolGenerator, List[BasePipeline]]. Add processing for PoolGenerator case in a code.

Test cases

• Add new test on running with custom PoolGenerator

Additional context

Blocked by #1269.

[Mr-Geekman]

It was decided to keep PoolGenerator class private and don't expose its API, because it isn't user-friendly.