Add a trainer.ckpt_path setter for stateful loading

[carmocca]

What does this PR do?

trainer = Trainer()
trainer.ckpt_path = "my/checkpoint/file.ckpt"
trainer.fit(model)

trainer.test()  # still uses the `ckpt_path` above

# you will be in charge of resetting this
trainer.ckpt_path = None
trainer.test()  # now uses `ckpt_path="best"` as it would normally

This is a tool that NeMo can use in replacement of the resume_checkpoint_path attribute, removed in #16167: https://github.com/NVIDIA/NeMo/blob/85cc023adc530de59dd13903c4541ca008b260d3/nemo/utils/exp_manager.py#L575

cc @Borda @justusschock @awaelchli @SeanNaren @titu1994

Does your PR introduce any breaking changes? If yes, please list them.

None

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🟢 pytorch_lightning: Tests workflow

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pl-cpu (windows-2022, pytorch, 3.10, 1.12)	success	✅
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pl-cpu (slow, macOS-11, pytorch, 3.7, 1.11)	success	✅
pl-cpu (slow, ubuntu-20.04, pytorch, 3.7, 1.11)	success	✅
pl-cpu (slow, windows-2022, pytorch, 3.7, 1.11)	success	✅
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These checks are required after the changes to src/pytorch_lightning/trainer/connectors/checkpoint_connector.py, src/pytorch_lightning/trainer/trainer.py, tests/tests_pytorch/core/test_datamodules.py, tests/tests_pytorch/trainer/connectors/test_checkpoint_connector.py, tests/tests_pytorch/trainer/test_trainer.py.

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install-pkg (macOS-12, fabric, 3.10)	success	✅
install-pkg (macOS-12, pytorch, 3.7)	success	✅
install-pkg (macOS-12, pytorch, 3.10)	success	✅
install-pkg (macOS-12, lightning, 3.7)	success	✅
install-pkg (macOS-12, lightning, 3.10)	success	✅
install-pkg (macOS-12, notset, 3.7)	success	✅
install-pkg (macOS-12, notset, 3.10)	success	✅
install-pkg (windows-2022, app, 3.7)	success	✅
install-pkg (windows-2022, app, 3.10)	success	✅
install-pkg (windows-2022, fabric, 3.7)	success	✅
install-pkg (windows-2022, fabric, 3.10)	success	✅
install-pkg (windows-2022, pytorch, 3.7)	success	✅
install-pkg (windows-2022, pytorch, 3.10)	success	✅
install-pkg (windows-2022, lightning, 3.7)	success	✅
install-pkg (windows-2022, lightning, 3.10)	success	✅
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These checks are required after the changes to src/pytorch_lightning/trainer/connectors/checkpoint_connector.py, src/pytorch_lightning/trainer/trainer.py.

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[awaelchli]

I don't understand these changes. We were supposed to make things simpler, not more complicated.