Pin diffusers<0.40 for the tf_min unit test env

The tf_min unit matrix pins `transformers~=4.57`, which requires
`huggingface_hub<1.0`. A fresh env's unbounded diffusers (`diffusers>=0.32.2`)
resolves to diffusers 0.40, which requires `huggingface_hub>=1.23` — an
unsatisfiable combination. The env keeps hub 0.36 (for transformers), so
diffusers 0.40's pipeline import fails (missing `get_cached_repo_tree`);
`is_diffusers_object` then returns False and diffusers models silently misroute
to the LLM export path, failing tests/unit/torch/export/test_export_diffusers.py
on tf_min (on main and every open PR).

Pin `diffusers<0.40` for the tf_min env (resolves to 0.39, which supports
`huggingface_hub<1.0`) so the env stays internally consistent. tf_latest is
unchanged. Validated locally: with the pin, the four failing diffusers export
tests pass under transformers 4.57.

Signed-off-by: Shengliang Xu <shengliangx@nvidia.com>

Author: shengliangxu

noxfile.py

@@ -42,9 +42,15 @@
     "torch_213": "torchvision~=0.28.0",
 }
 
+# Extra install pins applied per transformers matrix entry (installed after the base
+# ``.[all,dev-test]`` install to constrain that env).
 TRANSFORMERS_VERSIONS = {
-    "tf_latest": "transformers~=5.14.0",
-    "tf_min": "transformers~=4.57.0",
+    "tf_latest": ("transformers~=5.14.0",),
+    # transformers 4.57 caps ``huggingface_hub<1.0``, but ``diffusers>=0.40`` requires
+    # ``huggingface_hub>=1.23``. Bound diffusers to a hub<1.0-compatible release so this env
+    # stays internally consistent; otherwise diffusers' pipeline import fails and diffusers
+    # models silently misroute to the LLM path on export.
+    "tf_min": ("transformers~=4.57.0", "diffusers<0.40"),
 }
 
 
@@ -63,9 +69,9 @@ def _cov_args():
 def unit(session, torch_ver, tf_ver):
     """Unit tests — parametrized over torch and transformers versions."""
     session.install(TORCH_VERSIONS[torch_ver], "-e", ".[all,dev-test]")
-    tf_pin = TRANSFORMERS_VERSIONS[tf_ver]
-    if tf_pin:
-        session.install(tf_pin)
+    tf_pins = TRANSFORMERS_VERSIONS[tf_ver]
+    if tf_pins:
+        session.install(*tf_pins)
     session.run("python", "-m", "pytest", "tests/unit", *_cov_args(), env=_CPU_ONLY_ENV)