[DOC] Add usage examples for TimeXer and TemporalFusionTransformer

[atharva-mashalkar]

Reference Issues/PRs

Towards #2377 (adds the remaining v1 TimeXer and TemporalFusionTransformer items).

What does this implement/fix? Explain your changes.

Adds a runnable Examples section to the TimeXer and TemporalFusionTransformer class docstrings, in NumPy docstring style, following the merged xLSTMTime example: toy TimeSeriesDataSet, from_dataset(...), a fast_dev_run CPU Trainer, and predict.

TimeXer uses encoder length 16 and patch_length=8 so the context length is a multiple of the patch length.

The TFT class docstring already documents Parameters, so its Examples section is placed after that section.

What should a reviewer concentrate their feedback on?

• TimeXer encoder length and patch_length
• TemporalFusionTransformer hidden_size, attention_head_size, and hidden_continuous_size

Did you add any tests for the change?

No new tests; the examples are exercised by TestAllPtForecasters.test_doctest_examples.
Ran locally: pytest --doctest-modules on both model files (passed).

PR checklist

• The PR title starts with [DOC]
• Used pre-commit hooks on the changed files (pre-commit run --files)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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