fix: rich progress bar error when resume training

[deependujha]

What does this PR do?

Fixes #20976 #21015

Fixes an issue where the rich progress bar was not initialized when resuming training mid-epoch.

This occurred because on_train_epoch_start is not called during such resumes. The progress bar is now initialized in on_train_start to ensure correct behavior.

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📚 Documentation preview 📚: https://pytorch-lightning--21000.org.readthedocs.build/en/21000/

[Borda]

@deependujha, mind check the failing tests:

FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_leave[False-3] - assert 4 == 3
 +  where 4 = <function reset at 0x1496c07c0>.call_count
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_with_refresh_rate[4-6-6-5] - TypeError: The `RichProgressBar._trainer` reference has not been set yet.
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_reset_bars - TypeError: The `RichProgressBar._trainer` reference has not been set yet.
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_with_refresh_rate[3-1-0-1] - TypeError: The `RichProgressBar._trainer` reference has not been set yet.
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_with_refresh_rate[3-1-1-3] - TypeError: The `RichProgressBar._trainer` reference has not been set yet.
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_refresh_rate_disabled - AssertionError
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_custom_theme - TypeError: The `RichProgressBar._trainer` reference has not been set yet.
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_with_refresh_rate[1-2-3-6] - TypeError: The `RichProgressBar._trainer` reference has not been set yet.
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_with_refresh_rate[3-6-6-5] - TypeError: The `RichProgressBar._trainer` reference has not been set yet.
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_with_refresh_rate[3-5-0-2] - TypeError: The `RichProgressBar._trainer` reference has not been set yet.
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_with_refresh_rate[3-5-2-4] - TypeError: The `RichProgressBar._trainer` reference has not been set yet.
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_disabled - AssertionError
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_with_refresh_rate[7-6-6-3] - TypeError: The `RichProgressBar._trainer` reference has not been set yet.
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_with_refresh_rate[6-5-2-3] - TypeError: The `RichProgressBar._trainer` reference has not been set yet.
FAILED callbacks/progress/test_rich_progress_bar.py::test_rich_progress_bar_with_refresh_rate[1-0-0-0] - TypeError: The `RichProgressBar._trainer` reference has not been set yet.

[deependujha]

@Borda all tests pass now.