feat: add LTX2 smoke test and update setup.sh environment

[Perseus14]

Description

This PR adds an end-to-end smoke test for the LTX2 model in MaxDiffusion and updates the environment setup script to handle virtual environments more robustly.

Key Changes

1. LTX2 Smoke Test

• Added src/maxdiffusion/tests/ltx2_smoke_test.py to provide a basic verification that LTX2 generation runs successfully on the configured hardware. This follows similar patterns used for the Wan model tests.

2. Environment Setup (setup.sh)

• Package Index URLs: Explicitly set PIP_INDEX_URL and UV_INDEX_URL to https://pypi.org/simple to ensure standard resolution from PyPI.
• Conditional UV_SYSTEM_PYTHON: Instead of forcing uv to use the system Python by default, added a check to detect if the script is running within a virtual environment.
  • If in a virtual environment, it respects that environment and avoids setting UV_SYSTEM_PYTHON=1.
  • If using system Python, it sets UV_SYSTEM_PYTHON=1 to maintain compatibility with typical bare-metal TPU VM workflows where we want to use the system package space.

Verification Results

• Verified the virtual environment detection logic in setup.sh works as expected when run inside and outside a venv.
• The LTX2 smoke test has been added and is ready to be run in CI or manually.

[github-actions]

e2e testgrid: https://8bcf50593faf4ea38060e236169827e5-dot-us-central1.composer.googleusercontent.com/dags/maxdiffusion_tpu_e2e/grid