Fix: deterministically honor TORCH_CUDA_ARCH_LIST for the vLLM GPU arch.
Deriving CUDA_ARCHS from the torch wheel arch list is unreliable for a DGX Spark
build: the stock cu130 torch advertises sm_120, and the sm_121a-only FP4
instruction cvt.e2m1x2.f32 then fails in ptxas. When TORCH_CUDA_ARCH_LIST is set
(e.g. 12.1a), take the numeric arch (12.1) directly so the CUDA>=13 family
mapping rebuilds for sm_121a.

Note: this is one of two adjacent changes; see 0001-cuda13-supported-archs-add-12.1.patch.
---
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -221,6 +221,16 @@
   #
   clear_cuda_arches(CUDA_ARCH_FLAGS)
   extract_unique_cuda_archs_ascending(CUDA_ARCHS "${CUDA_ARCH_FLAGS}")
+  # Deterministically honor TORCH_CUDA_ARCH_LIST (e.g. "12.1a" -> 12.1)
+  # instead of deriving from the torch wheel arch list. Required so a
+  # DGX Spark (GB10/sm_121a) build targets the 12.1 family; otherwise the
+  # sm_120 FP4 cvt.e2m1x2.f32 path fails in ptxas.
+  if(DEFINED ENV{TORCH_CUDA_ARCH_LIST} AND NOT "$ENV{TORCH_CUDA_ARCH_LIST}" STREQUAL "")
+    string(REGEX MATCH "[0-9]+\.[0-9]+" _forced_arch "$ENV{TORCH_CUDA_ARCH_LIST}")
+    if(NOT _forced_arch STREQUAL "")
+      set(CUDA_ARCHS "${_forced_arch}")
+    endif()
+  endif()
   message(STATUS "CUDA target architectures: ${CUDA_ARCHS}")
   # Filter the target architectures by the supported supported archs
   # since for some files we will build for all CUDA_ARCHS.