[MNT] isolate `pytorch-optimizer` as soft dependency

[fkiraly]

Isolates pytorch-optimizer as soft dependency in a new soft dep set all_extras. https://github.com/jdb78/pytorch-forecasting/issues/1616

The imports happen only in BaseModel, when resolving aliases for optimizer.

Isolation consists of two steps:

• replacing resolution of the alias with a request to install the package, and removing from resolution scope if not installed
• replacing the default optimizer "ranger" with "adam" if pytorch-optimizer is not installed (left at "ranger" if installed)

Deprecation messages and actions are added, to changne the default to "adam" from 1.2.0, in order to minimize the number of dependencies in default parameter settings

[codecov-commenter]

:warning: Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

Attention: Patch coverage is 45.00000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 90.14%. Comparing base (95fa06c) to head (38daba9).

Files with missing lines	Patch %	Lines
pytorch_forecasting/models/base_model.py	45.00%	11 Missing :warning:

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1641 +/- ## ========================================== + Coverage 90.10% 90.14% +0.04% ========================================== Files 32 32 Lines 4768 4780 +12 ========================================== + Hits 4296 4309 +13 + Misses 472 471 -1 ``` | [Flag](https://app.codecov.io/gh/jdb78/pytorch-forecasting/pull/1641/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Beitner) | Coverage Δ | | |---|---|---| | [cpu](https://app.codecov.io/gh/jdb78/pytorch-forecasting/pull/1641/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Beitner) | `90.14% <45.00%> (+0.04%)` | :arrow_up: | | [pytest](https://app.codecov.io/gh/jdb78/pytorch-forecasting/pull/1641/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Beitner) | `90.14% <45.00%> (+0.04%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Beitner#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.