Fix issue breaking cudagraphs (#685)

@ngimel figured this one out. 

The errors we were seeing from cudagraphs capture were coming from
`cuStreamGetCtx` which is not allowed while a stream is capturing.

It appears the result of `cuStreamGetCtx()` isn't even used, so I
believe it can just be removed.

python/triton/compiler.py

@@ -1026,13 +1026,10 @@ static inline void init_module(CUdevice device) {{
 
 
 void _{kernel_name}(int gridX, int gridY, int gridZ, CUstream stream, {arg_decls}) {{
-  CUcontext ctx;
-  CUdevice device;
-  CUDA_CHECK(cuStreamGetCtx(stream, &ctx));
-  CUDA_CHECK(cuCtxGetDevice(&device));
-
   // TODO: machine may have heterogeneous devices
   if(function == 0){{
+    CUdevice device;
+    CUDA_CHECK(cuCtxGetDevice(&device));
     init_module(device);
   }}
   void *params[] = {{ {', '.join(f"&arg{i}" for i in signature.keys() if i not in constants)} }};