[TEMP - do not merge] Force PR validation to test Python 3.15

Temporary commit to exercise the 3.15 conda-forge validation path on this PR:
- generate_binary_build_matrix.py: nightly python arches -> [3.15, 3.15t]
  (PR limit picks the first, so the PR validates 3.15 across linux arches).
- validate-nightly-binaries.yml: scope to os=linux and torchonly=true
  (no torchvision cp315 wheels yet).

REVERT THIS COMMIT before merging #8216.

Author: atalman

.github/workflows/validate-nightly-binaries.yml

@@ -32,4 +32,7 @@ jobs:
     uses: ./.github/workflows/validate-binaries.yml
     with:
       channel: nightly
-      os: all
+      # TEMPORARY (revert before merge): scope to linux and torch-only to test
+      # the Python 3.15 validation path (no torchvision cp315 wheels yet).
+      os: linux
+      torchonly: true

tools/scripts/generate_binary_build_matrix.py

@@ -22,7 +22,9 @@
 
 
 PYTHON_ARCHES_DICT = {
-    "nightly": ["3.10", "3.11", "3.12", "3.13", "3.14", "3.14t"],
+    # TEMPORARY (revert before merge): restrict nightly to 3.15/3.15t so this
+    # PR's nightly validation exercises the Python 3.15 conda-forge setup.
+    "nightly": ["3.15", "3.15t"],
     "test": ["3.10", "3.11", "3.12", "3.13", "3.14", "3.14t"],
     "release": ["3.10", "3.11", "3.12", "3.13", "3.14", "3.14t"],
 }