Fix no-std feature configuration to auto-fallback from std to alloc

[antimora]

This PR addresses issue #650 by following the convention of the feature configuration for no_std support.

Changes

• Removed mutual exclusivity between std and alloc features: Previously, the crate required exactly one of std or alloc
  to be enabled, causing compilation errors when both or neither were specified
• Automatic alloc fallback: When std is not enabled, the crate now automatically uses alloc + hashbrown::HashMap
• Backward compatibility: The alloc feature is retained as a no-op for compatibility with existing downstream crates

How it works

• Default build (cargo build): Uses std with std::collections::HashMap
• No-std build (cargo build --no-default-features): Automatically uses alloc with hashbrown::HashMap
• Explicit alloc (cargo build --no-default-features --features alloc): Still works for backward compatibility

Testing

All tests pass in both configurations:

cargo test                          # std mode ✓
cargo test --no-default-features    # no-std mode ✓
cargo test --no-default-features --features alloc  # backward compat ✓

This aligns with common Rust ecosystem patterns where default = ["std"] provides standard library support, and
--no-default-features automatically provides no_std support with alloc.

Fixes #650

[antimora]

Hi @Narsil, this is to support both std/no_std. The feature flag patten follows "std" flag convention. This is needed for Burn project to support no_std when "std" is disabled by no-default-features. I tested locally and it works.

[Narsil]

LGTM. I'm not sure why we wrote it being exclusive features in the first place.

Thanks.

[antimora]

LGTM. I'm not sure why we wrote it being exclusive features in the first place.

Thanks.

yeah, it's rare that mutually exclusive feature flags are needed. Probably at the type the pattern wasn't known.

BTW, I updated no-std test command in CI workflow to remove alloc flag. Just case if someone accidentally re-introduces the alloc flag usage.

[antimora]

@Narsil , thank you for the review and merge. I checked feature configurations and no-std works for Burn.

Do you know if there is a chance to release the patch in near future? The Burn team would like to make a release soon. I am currently linking github repo in Cargo.toml