remove gradcheck import for torch<1.7

MinkowskiEngine/utils/__init__.py

@@ -23,6 +23,5 @@
 # of the code.
 from .quantization import sparse_quantize, ravel_hash_vec, fnv_hash_vec
 from .collation import SparseCollation, batched_coordinates, sparse_collate, batch_sparse_collate
-from .gradcheck import gradcheck
 from .coords import get_coords_map
 from .init import kaiming_normal_

README.md

@@ -301,6 +301,11 @@ For generative transposed convolution, please cite:
 }
 ```
 
+
+## Unittest
+
+For unittests and gradcheck, use torch >= 1.7
+
 ## Projects using Minkowski Engine
 
 Please feel free to update [the wiki page](https://github.com/NVIDIA/MinkowskiEngine/wiki/Usage) to add your projects!