pydantic v2 migration

[kashif]

Initial pydantic v2 migraton

[kashif]

@ddelange can you help me fix up the tests?

[ddelange]

/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/gluonts/ext/rotbaum/_types.py:28: in FeatureImportanceResult
    @model_validator()
E   TypeError: model_validator() missing 1 required keyword-only argument: 'mode'

mode: Specifies whether to validate the fields before or after validation.

https://docs.pydantic.dev/latest/api/functional_validators

[ddelange]

test/core/test_serde_dataclass.py:22: in <module>
    class Estimator:
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/gluonts/core/serde/_dataclass.py:139: in dataclass
    return _dataclass(cls, init, repr, eq, order, unsafe_hash, frozen)
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/gluonts/core/serde/_dataclass.py:155: in _dataclass
    pydantic.dataclasses.dataclass(
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/pydantic/dataclasses.py:141: in dataclass
    assert init is False, 'pydantic.dataclasses.dataclass only supports init=False'
E   AssertionError: pydantic.dataclasses.dataclass only supports init=False

https://github.com/awslabs/gluonts/actions/runs/6611474968/job/17955418303?pr=3007#step:5:75

[ddelange]

E       pydantic_core._pydantic_core.ValidationError: 1 validation error for CategoricalFeatureInfo
E       cardinality
E         Input should be a valid string [type=string_type, input_value=10, input_type=int]
E           For further information visit https://errors.pydantic.dev/2.4/v/string_type

https://github.com/awslabs/gluonts/actions/runs/6611474968/job/17955418303?pr=3007#step:5:701

[ddelange]

Hi @kashif 👋

I'm trying a less intrusive approach in #3026