feat: apply_runtime_settings — permanent runtime config for AOT-loaded engines

[tp5uiuc]

Description

Adds torch_tensorrt.runtime.apply_runtime_settings(target, settings), a
public API to permanently configure runtime settings on engines loaded via
torch_tensorrt.load() (AOT-loaded artifacts with no TorchTensorRTModule
wrapper).

What — A new entry point that applies a RuntimeSettings object to every
TRT engine reachable from an nn.Module, a list of modules, or an
ExportedProgram. Returns the number of engines updated.

Why — The existing runtime_config and runtime_cache context managers
auto-restore on exit and anchor to TorchTensorRTModule; they cannot reach
engines produced by the AOT save/load path. There was no supported way to
attach a RuntimeCache (or any other setting) to a bare loaded artifact.

How

• py/torch_tensorrt/runtime/_runtime_config.py: add _is_trt_engine,
  _iter_trt_engines, _send_settings_to_engine, and apply_runtime_settings;
  update both context managers to use _iter_trt_engines and raise TypeError
  on module-less engines (naming apply_runtime_settings in the message).
• py/torch_tensorrt/dynamo/runtime/_TorchTensorRTModule.py: reduce
  _send_to_engine to a one-line delegate to _send_settings_to_engine.
• py/torch_tensorrt/runtime/__init__.py: export apply_runtime_settings.
• docsrc/py_api/runtime.rst + docsrc/user_guide/runtime_performance/runtime_settings.rst:
  add API entry and AOT section with example and caller-ownership rules.
• tests/py/dynamo/runtime/test_005_apply_runtime_settings.py: new test file
  covering type errors, module-owned engines, module-less engines, mixed targets,
  CM raises, and warm-load byte-count verification.
• tests/py/dynamo/runtime/test_{000,001,004}_*.py,
  tests/py/dynamo/models/test_*_models.py: replace local _apply_runtime_settings
  helpers with the public API.

Key constraints applied: string runtime_cache paths are rejected for
module-less engines (caller owns the handle and must call cache.load() before
applying); ExportedProgram supported via ep.constants; apply is
validate-before-mutate (all-or-nothing across a mixed target list).

Testing

Full tests/py/dynamo/runtime/ suite and the three model test files pass on
both the Python-only and C++ runtime builds. New test file adds 11 tests; all
green.

Note: this PR stacks on #4482 (fix/python-runtime-cache-on-cpp-build);
those commits appear in the diff until #4482 merges.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project (You can use the linters)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas and hacks
• I have made corresponding changes to the documentation
• I have added tests to verify my fix or my feature
• New and existing unit tests pass locally with my changes