fix(trainer): simplify local-exec command building process

[khushiiagrawal]

What this PR does / why we need it:

Different from the previous approach of using complex Bash script templates, this PR simplifies the local-exec command building process by introducing a generated Python runner script (runner.py). This improves maintainability, debugging, and cross-platform compatibility.

Specific changes include:

• Refactored Command Generation: Replaced LOCAL_EXEC_JOB_TEMPLATE (Bash) with RUNNER_TEMPLATE (Python) in constants.py.
• Updated Utils: Modified utils.py to generate the runner.py script which handles virtual environment creation, dependency installation, and training command execution using Python's subprocess.
• Bug Fix: Corrected LocalProcessBackend.__get_job_status in backend.py to properly return TRAINJOB_COMPLETE instead of defaulting to TRAINJOB_CREATED, ensuring wait_for_job_status functions correctly.
• Testing: Added utils_test.py to verify the command generation logic.

Fixes #93

Checklist:

• Docs included if any changes are user facing

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[khushiiagrawal]

hii @kramaranya @astefanutti

I noticed that the uv.lock file in the main branch seems to be corrupted. When I try to run the tests locally to verify my changes, uv lock --check fails with a syntax error (missing field version at line 921).

I haven't included the lockfile regeneration in this commit yet, but as a result, the tests are currently failing locally (and likely in CI if it checks the lockfile).

Could you please confirm if I should regenerate and commit the fixed uv.lock in this PR ? This seems necessary to get the tests passing.
Thanks!

[khushiiagrawal]

@andreyvelich @akshaychitneni Please take a look.
Thanks!