fix: path-string runtime cache freed itself out from under the engine

[tp5uiuc]

What

A runtime_cache passed as a path string freed itself out from under the engine: the next createExecutionContext ran against a destroyed IRuntimeCache and segfaulted.

Why

TRTRuntimeConfig is documented as owning the engine-implicit RuntimeCache, but had no slot for it — _apply_settings built it as a local with autosave_on_del=True, so it was collected the moment the method returned, taking the live IRuntimeCache with it.

TorchTensorRTModule pre-wraps path strings into a module-owned handle, so engines reached through compile() never take this branch. Engines built straight from packed engine info do: they carry the default RuntimeSettings(runtime_cache=RUNTIME_CACHE_PATH) with no module around them to resolve it. As a side effect the disk cache was never populated on this path — autosave fired before the engine had generated a kernel.

How

1. Guard — ensure_cache() returns None when the facade is torchbind-backed. No library path reaches that (every in-library TRTEngine construction site is gated on the same flag that picks the backing), but a hand-built engine on a build shipping libtorchtrt.so does, and set_runtime_cache rejects an explicitly-passed None despite advertising cache: IRuntimeCache = None. Skip the attach and warn instead of surfacing a pybind signature error; the two duplicated call sites fold into one helper.
2. Lifetime — hold the implicit wrapper on the config, reused across reset() when the path is unchanged so a settings swap does not discard in-memory kernels. Mirrors what TorchTensorRTModule._resolve_runtime_cache already does for module-owned handles.

TestPythonRuntimeAliasedIO is skipped when the C++ runtime is available: it hand-builds a Python engine, which is a configuration the library never produces there. It stays live on python-only builds, matching the existing guard in test_000_runtime_cache.py and test_001_dynamic_shapes_kernel_strategy.py.

Testing

Build	Result
C++ runtime, full tests/py/dynamo/runtime/	169 passed, 51 skipped
python-only TRT-RTX, runtime-cache + aliased-IO	38 passed, 1 skipped
python-only TRT-RTX, before this change	SIGSEGV in createExecutionContext

The crash is caught by test_bare_engine_executes_with_default_path_cache. On main, TestPythonRuntimeAliasedIO already fails on python-only TRT-RTX for the same root cause (Failed to create execution context), so this is pre-existing rather than introduced here.

Type of change

• Bug fix (non-breaking change which fixes an issue)

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
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