Put the onnx example behind a feature flag. (#1276)

* Put the onnx example behind a feature flag.

* Exclude the onnx bits from the workspace.

* README tweaks.

README.md

@@ -221,6 +221,7 @@ Cheatsheet:
 - [candle-datasets](./candle-datasets/): Datasets and data loaders.
 - [candle-transformers](./candle-transformers): transformers-related utilities.
 - [candle-flash-attn](./candle-flash-attn): Flash attention v2 layer.
+- [candle-onnx](./candle-onnx/): ONNX model evaluation.
 
 ## FAQ
 
@@ -348,25 +349,6 @@ mdbook test candle-book -L .\target\debug\deps\ `
 This may be caused by the models being loaded from `/mnt/c`, more details on
 [stackoverflow](https://stackoverflow.com/questions/68972448/why-is-wsl-extremely-slow-when-compared-with-native-windows-npm-yarn-processing).
 
-#### Missing protoc installation when compiling candle-onnx
-
-The candle-onnx dependency prost-build no longer comes bundled with prost binaries. This could cause the following error when attempting to compile candle-onnx:
-
-```
-error: failed to run custom build command for `candle-onnx v0.3.0 (C:\Projekt\candle\candle-onnx)`
-Caused by: // (...)
-  Could not find `protoc` installation and this build crate cannot proceed without
-      this knowledge.
-```
-
-To fix this issue:
-
-1. Download protoc for your platform from the official Protocol Buffers GitHub repository: [github.com/protocolbuffers/protobuf/releases](https://github.com/protocolbuffers/protobuf/releases).
-2. Extract the protoc binary from the downloaded archive's bin folder.
-3. Add the location of the protoc binary to your system's PATH, or set the location in a new PROTOC environment variable.
-
-This should resolve the error and allow you to compile candle-onnx successfully.
-
 #### Tracking down errors
 
 You can set `RUST_BACKTRACE=1` to be provided with backtraces when a candle