[test] add LTX-2 distilled T2V SSIM regression test

LTX-2 was the only model family in fastvideo/pipelines/basic/ without an
SSIM coverage file. Add test_ltx2_similarity.py alongside the other
per-family SSIM tests so the in-flight API refactor and future
LTX-2-specific changes (two-stage refine, gpu_pool upstream, Dynamo
backend) have a golden-quality regression guard.

Parameters:

  Default (CI-friendly):
    model: FastVideo/LTX2-Distilled-Diffusers (8-step distilled)
    resolution: 512x768, num_frames=45, num_inference_steps=4
    sp_size=2 on 2 GPUs, FLASH_ATTN backend
    ltx2_vae_tiling=True for peak-memory safety

  --ssim-full-quality:
    falls back to the ltx2_distilled preset defaults
    (1024x1536, 121 frames, 8 steps, guidance_scale=1.0)

Uses the shared run_text_to_video_similarity_test helper (same pattern
as Wan/TurboDiffusion), so _build_init_kwargs picks up
ltx2_vae_tiling + related tile sizes automatically.

REQUIRED_GPUS = 2 is declared at module scope so the Modal SSIM
orchestrator (fastvideo/tests/modal/ssim_test.py) schedules it
correctly on the L40S:8 runner. LTX2_DISTILLED_MODEL_TO_PARAMS is
named so the orchestrator can split by model id (one subprocess per
entry) for future multi-model LTX-2 coverage.

Threshold min_acceptable_ssim=0.93 matches Wan T2V.

Reference videos are not in the repo. After this lands on main, run
the test once on an L40S and upload via
`python fastvideo/tests/ssim/reference_videos_cli.py upload --quality-tier all`
to seed FastVideo/ssim-reference-videos. Subsequent runs (including
regression guards for in-flight refactor PRs) auto-download the
references before the test executes.

fastvideo/tests/ssim/test_ltx2_similarity.py

@@ -0,0 +1,93 @@
+# SPDX-License-Identifier: Apache-2.0
+"""SSIM-based similarity test for LTX-2 distilled text-to-video.
+
+Parameters derived from examples/inference/basic/basic_ltx2_distilled.py,
+with resolution + num_inference_steps reduced to keep GPU CI runtime
+bounded. Full-quality variant (via ``--ssim-full-quality``) falls back
+to the ``ltx2_distilled`` preset defaults.
+"""
+import os
+
+import pytest
+
+from fastvideo.api.sampling_param import SamplingParam
+from fastvideo.logger import init_logger
+from fastvideo.tests.ssim.inference_similarity_utils import (
+    resolve_inference_device_reference_folder,
+    run_text_to_video_similarity_test,
+)
+
+logger = init_logger(__name__)
+
+REQUIRED_GPUS = 2
+
+device_reference_folder = resolve_inference_device_reference_folder(logger)
+
+LTX2_DISTILLED_PARAMS = {
+    "num_gpus": 2,
+    "model_path": "FastVideo/LTX2-Distilled-Diffusers",
+    "height": 512,
+    "width": 768,
+    "num_frames": 45,
+    "num_inference_steps": 4,
+    "guidance_scale": 1.0,
+    "seed": 10,
+    "sp_size": 2,
+    "tp_size": 1,
+    "fps": 24,
+    "ltx2_vae_tiling": True,
+}
+_LTX2_DISTILLED_FULL_QUALITY_DEFAULTS = SamplingParam.from_pretrained(
+    LTX2_DISTILLED_PARAMS["model_path"])
+LTX2_DISTILLED_FULL_QUALITY_PARAMS = {
+    "num_gpus": LTX2_DISTILLED_PARAMS["num_gpus"],
+    "model_path": LTX2_DISTILLED_PARAMS["model_path"],
+    "height": _LTX2_DISTILLED_FULL_QUALITY_DEFAULTS.height,
+    "width": _LTX2_DISTILLED_FULL_QUALITY_DEFAULTS.width,
+    "num_frames": _LTX2_DISTILLED_FULL_QUALITY_DEFAULTS.num_frames,
+    "num_inference_steps":
+        _LTX2_DISTILLED_FULL_QUALITY_DEFAULTS.num_inference_steps,
+    "guidance_scale": _LTX2_DISTILLED_FULL_QUALITY_DEFAULTS.guidance_scale,
+    "seed": _LTX2_DISTILLED_FULL_QUALITY_DEFAULTS.seed,
+    "sp_size": LTX2_DISTILLED_PARAMS["sp_size"],
+    "tp_size": LTX2_DISTILLED_PARAMS["tp_size"],
+    "fps": _LTX2_DISTILLED_FULL_QUALITY_DEFAULTS.fps,
+    "ltx2_vae_tiling": LTX2_DISTILLED_PARAMS["ltx2_vae_tiling"],
+}
+
+LTX2_DISTILLED_MODEL_TO_PARAMS = {
+    "LTX2-Distilled-Diffusers": LTX2_DISTILLED_PARAMS,
+}
+FULL_QUALITY_LTX2_DISTILLED_MODEL_TO_PARAMS = {
+    "LTX2-Distilled-Diffusers": LTX2_DISTILLED_FULL_QUALITY_PARAMS,
+}
+
+LTX2_DISTILLED_TEST_PROMPTS = [
+    "A warm sunny backyard. The camera starts in a tight cinematic "
+    "close-up of a woman and a man in their 30s, facing each other with "
+    "serious expressions. The camera slowly pans right, revealing a "
+    "grandfather in the garden wearing enormous butterfly wings, waving "
+    "his arms in the air like he's trying to take off. The tone is "
+    "deadpan, absurd, and quietly tragic.",
+]
+
+
+@pytest.mark.parametrize("prompt", LTX2_DISTILLED_TEST_PROMPTS)
+@pytest.mark.parametrize("attention_backend_name", ["FLASH_ATTN"])
+@pytest.mark.parametrize("model_id", list(LTX2_DISTILLED_MODEL_TO_PARAMS.keys()))
+def test_ltx2_distilled_inference_similarity(
+    prompt: str,
+    attention_backend_name: str,
+    model_id: str,
+) -> None:
+    run_text_to_video_similarity_test(
+        logger=logger,
+        script_dir=os.path.dirname(os.path.abspath(__file__)),
+        device_reference_folder=device_reference_folder,
+        prompt=prompt,
+        attention_backend_name=attention_backend_name,
+        model_id=model_id,
+        default_params_map=LTX2_DISTILLED_MODEL_TO_PARAMS,
+        full_quality_params_map=FULL_QUALITY_LTX2_DISTILLED_MODEL_TO_PARAMS,
+        min_acceptable_ssim=0.93,
+    )