Add some preliminary ONNX support (#1260)

* Add the onnx protos.

* Move the reading bits.

* Install protoc on the CI.

* Install protoc on the cuda CI too.

* Use clap for the onnx tool.

* Tweak the CI protoc install.

* Add some simple evalution function.

* Add some binary operator support.

candle-onnx/build.rs

@@ -0,0 +1,6 @@
+use std::io::Result;
+
+fn main() -> Result<()> {
+    prost_build::compile_protos(&["src/onnx.proto3"], &["src/"])?;
+    Ok(())
+}