Pin torchmetrics. (#1123)

`torchmetrics` 0.7.0 released yesterday (Jan 17th 2022) is causing an issue with pytorch_lightning: Lightning-AI/pytorch-lightning#11524

Author: rosbo

Dockerfile.tmpl

@@ -483,7 +483,8 @@ RUN pip install flashtext && \
     pip install vowpalwabbit && \
     pip install pydub && \
     pip install pydegensac && \
-    pip install torchmetrics && \
+    # b/215182966 torchmetrics 0.7.0 is causing an issue with pytorch-lightning.
+    pip install torchmetrics==0.6.2 && \
     pip install pytorch-lightning && \
     pip install datatable && \
     pip install sympy && \