feat: Add train() and optimize() methods to TrainJobTemplate

[sujalshah-bit]

What this PR does / why we need it:
TrainJobTemplate was a passive data container with no way to actually execute jobs. This change lets the template act as an entrypoint to both TrainerClient and OptimizerClient.

• train() delegates to TrainerClient.train() using the template's pre-configured runtime, initializer, and trainer
• optimize() delegates to OptimizerClient.optimize() passing self as the trial_template for hyperparameter tuning

TYPE_CHECKING is used to avoid circular imports since TrainerClient and OptimizerClient both already import types.py.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):

Fixes NONE

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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[github-actions]

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[Copilot]

Pull request overview

This PR turns TrainJobTemplate from a passive configuration object into an executable entrypoint by adding convenience methods that delegate to TrainerClient and OptimizerClient.

Changes:

• Added TrainJobTemplate.train() to submit a TrainJob via an injected TrainerClient.
• Added TrainJobTemplate.optimize() to submit an OptimizationJob via an injected OptimizerClient (using the template as trial_template).
• Introduced postponed annotation evaluation (from __future__ import annotations) plus TYPE_CHECKING imports to avoid runtime circular imports.

[Copilot]

The constant assignment TORCH_TUNE = TORCH_TUNE = ... is redundant and looks accidental; simplify to a single assignment to avoid confusion and potential lint issues.

Suggested change

	TORCH_TUNE = TORCH_TUNE = TorchTuneConfig.__name__.lower().replace("config", "")
	TORCH_TUNE = TorchTuneConfig.__name__.lower().replace("config", "")

[Copilot]

The TODO comment above TrainJobTemplate about adding train()/optimize() is now outdated after introducing these methods; remove or update it to avoid misleading future readers.