Bump torch to >=1.9.0 (#522)

This is necessary for #521 to work. The minimal version where `torch.inference_mode()` works is 1.9.0.

requirements.txt

@@ -1,5 +1,5 @@
 PyYAML
-torch>=1.6.0
+torch>=1.9.0
 numpy>=1.17
 scipy>=1.2.1
 prefetch_generator>=1.0.1