Try PyTorch nightly for GB10/sm_121 GPU support

The NVIDIA GB10 (Grace Blackwell) requires sm_121 compute capability
which isn't in stable PyTorch yet. Nightly may have it.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker/Dockerfile.spark

@@ -13,11 +13,11 @@ RUN apt-get update && apt-get install -y \
 
 WORKDIR /app
 
-# Install PyTorch with CUDA support first (before other requirements)
-# Using CUDA 12.4 wheels which are compatible with CUDA 13.0 driver
-RUN pip3 install --no-cache-dir \
+# Install PyTorch nightly with CUDA support (for newer GPU architectures like GB10/sm_121)
+# Using nightly builds which may have support for newer compute capabilities
+RUN pip3 install --no-cache-dir --pre \
     torch \
-    --index-url https://download.pytorch.org/whl/cu124
+    --index-url https://download.pytorch.org/whl/nightly/cu124
 
 # Copy requirements and install (excluding packages with ARM64 issues)
 COPY requirements.txt .