catch error importing blocksparse tensor when triton not installed (f…

…acebookresearch#213)

Co-authored-by: Diana Liskovich <dianaml@devfair0471.h2.fair>

xformers/sparse/__init__.py

@@ -3,6 +3,13 @@
 # This source code is licensed under the BSD license found in the
 # LICENSE file in the root directory of this source tree.
 
+import logging
+
+try:
+    from .blocksparse_tensor import BlockSparseTensor  # noqa: F401
+except ImportError as e:
+    logging.warning(
+        f"Triton is not available, some optimizations will not be enabled.\nError {e}"
+    )
 
-from .blocksparse_tensor import BlockSparseTensor  # noqa: F401
 from .csr_tensor import SparseCSRTensor  # noqa: F401