Fix py-torch build

PyTorch 1.9.1 (fixed in versions >= 1.10.0) depends on an older onnx version that utilizes a deprecated function from Google's protobuf library that was removed in version 3.18.0 (see https://github.com/protocolbuffers/protobuf/pull/8794/).

Workaround by specifying an explicit version range for this dependency.