Improve the ONNX basic example + bugfixes (#1266)

* Generate some zeros tensor in the onnx simple-eval example.

* Fix the casting operation.

* Support more ops.

* Handle reshape.

* Concat.

* Softmax.

candle-onnx/src/lib.rs

@@ -6,7 +6,7 @@ pub mod onnx {
 }
 
 mod eval;
-pub use eval::simple_eval;
+pub use eval::{dtype, simple_eval};
 
 pub fn read_file<P: AsRef<std::path::Path>>(p: P) -> Result<onnx::ModelProto> {
     let buf = std::fs::read(p)?;