Split autodiff into autodiff_forward and autodiff_reverse

[Sa4dUs]

This PR splits #[autodiff] macro so #[autodiff(df, Reverse, args)] would become #[autodiff_reverse(df, args)] and #[autodiff(df, Forward, args)] would become #[autodiff_forwad(df, args)].

Pending tasks:

• Fix cannot find a built-in macro with name error.
• Update the tests to reflect the new macro structure.
• Review the implementation for better code.

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[ZuseZ4]

I'd almost bet it has to do with bootstrapping, see e.g. https://github.com/rust-lang/rust/pull/129458/files#diff-ea30f57e78f3b861b784ae315b3ac31358b7cea2aa14a668084ad412de12e586
It will be removed by other people later after landing, that's probably why you didn't see it anymore when just looking at the current code for it. Ping me if that doesn't solve the issue, then I can try to play around with it locally.

[jieyouxu]

I don't remember exactly, can built-in attributes be namespaced now? E.g. #[autodiff::forward()] and #[autodiff::reverse()]. That way, only one "top-level" name would cause macro ambiguities from the built-in attribute (autodiff), instead of two.

[ZuseZ4]

I think :: looks much prettier than _. Our attribute is called rustc_autodiff and not autodiff, so I don't think there will be ambiguities either way.
@jdonszelmann is probably the right person to ask about parsing. (see the question above)

[jieyouxu]

Right sorry, I keep forgetting there's two