Install a pinned version of onnx (#1004)

http://b/185992410

Author: Philmod

Dockerfile

@@ -386,6 +386,9 @@ RUN pip install flashtext && \
     pip install https://github.com/hbasria/ggpy/archive/0.11.5.zip && \
     pip install cesium && \
     pip install rgf_python && \
+    # b/185992410: onnx is a dependency of pytext, but the version 1.9.0 breaks pytext test.
+    # Remove this installation when pytext fixes the problem.
+    pip install onnx==1.8.1 && \
     # b/145404107: latest version force specific version of numpy and torch.
     pip install pytext-nlp==0.1.2 && \
     pip install tsfresh && \