Fail to build examples/mnist-inference-web

[tiruka]

Describe the bug

When building wasm to follow https://github.com/tracel-ai/burn/tree/main/examples/mnist-inference-web, it fails to build it as follows.

$ ./build-for-web.sh wgpu
error: lto cannot be used for `proc-macro` crate type without `-Zdylib-lto`
error: could not compile `zerocopy-derive` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `walrus-macro` (lib) due to 1 previous error
error: could not compile `serde_derive` (lib) due to 1 previous error
error: failed to compile `wasm-bindgen-cli v0.2.92`, intermediate artifacts can be found at `/var/folders/sp/srxtqmsj43q17d8vhfjsbrdm0000gn/T/cargo-installeGnqzX`.
To reuse those artifacts with a future compilation, set the environment variable `CARGO_TARGET_DIR` to that path.
Error: Installing wasm-bindgen with cargo
Caused by: Installing wasm-bindgen with cargo
Caused by: failed to execute `cargo install`: exited with exit status: 101
  full command: "cargo" "install" "--force" "wasm-bindgen-cli" "--root" "/Users/Library/Caches/.wasm-pack/.wasm-bindgen-cargo-install-0.2.92" "--version" "0.2.92"

To Reproduce

$ ./build-for-web.sh wgpu

When targeting ndarray, the result is the same, causes the same error.

Expected behavior

Successfully built.

Desktop (please complete the following information):

• OS: macOS 14.4.1

Additional context

To resolve this issue, I propose removing the -C lto=fat option from RUSTFLAGS. This change will ensure compatibility with proc-macro crates and prevent the build errors.
The link in the rust indicates lto cannot be used for proc-macro crate type without -Zdylib-lto, and -Zdylib-lto is not included in the standard rustc, the way I see it.

[tiruka]

Merged, so close this issue.