fix: pin version of `torchmetrics` for docs builds
The just-released 0.5.1 appears to raise an error on import, at least in
the context of docs builds, so pin it for now.
# pin docutils at 0.15.2 to avoid transitive dependency conflict with botocore (requires < 0.16)
docutils==0.15.2
+# 0.5.1 raises an error in import during docs bulids.
+torchmetrics==0.5.0
pillow
pytorch-lightning==1.3.5
