Make mypy checks opt-out instead of opt-in, fix several type issues

[lostella]

Description of changes:

• Fix several type issues through the package
• Mark many other failing checks as "ignore", either because there's too much code to go through otherwise (rotbaum), or because it would be impractical to ensure checks passing
• Update setup.py to use an opt-out mechanism for type checking, instead of opt-in
• Remove all .typesafe files, add a .typeunsafe file to gluonts.nursery to opt it out
• Marking as BREAKING because some code moved from gluonts.torch.distributions to gluonts.torch.model.mqf2

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[lostella]

Todos:

• get rid of a circular import I introduced (may need to break some import path there, but for good reasons)
• solve mypy complaints about "duplicate modules" (see following, also here)

src/gluonts/torch/model/deep_npts/__init__.py: error: Duplicate module named "gluonts.torch.model.deep_npts" (also at "/home/runner/work/gluonts/gluonts/src/gluonts/torch/model/deep_npts/__init__.py")
src/gluonts/torch/model/deep_npts/__init__.py: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#mapping-file-paths-to-modules for more info
src/gluonts/torch/model/deep_npts/__init__.py: note: Common resolutions include: a) using `--exclude` to avoid checking one of them, b) adding `__init__.py` somewhere, c) using `--explicit-package-bases` or adjusting MYPYPATH

• make the type-checking mechanism opt-out, and opt-out problematic packages in gluonts.nursery