Supported RUSTFLAGS for Enzyme

# Supported `RUSTFLAGS` To support you while debugging or profiling, we have added support for an experimental `-Z autodiff` rustc flag (which can be passed to cargo via `RUSTFLAGS`), which allow changing the behaviour of Enzyme, without recompiling rustc. We currently support the following values for `autodiff`. ### Debug Flags ```text PrintTA // Print TypeAnalysis information PrintAA // Print ActivityAnalysis information Print // Print differentiated functions while they are being generated and optimized PrintPerf // Print AD related Performance warnings PrintModBefore // Print the whole LLVM-IR module directly before running AD PrintModAfter // Print the whole LLVM-IR module after running AD, before optimizations PrintModFinal // Print the whole LLVM-IR module after running optimizations and AD LooseTypes // Risk incorrect derivatives instead of aborting when missing Type Info ``` <div class="warning"> `LooseTypes` is often helpful to get rid of Enzyme errors stating `Can not deduce type of <X>` and to be able to run some code. But please keep in mind that this flag absolutely has the chance to cause incorrect gradients. Even worse, the gradients might be correct for certain input values, but not for others. So please create issues about such bugs and only use this flag temporarily while you wait for your bug to be fixed. </div> ### Benchmark flags For performance experiments and benchmarking we also support ```text NoPostopt // We won't optimize the LLVM-IR Module after AD RuntimeActivity // Enables the runtime activity feature from Enzyme Inline // Instructs Enzyme to maximize inlining as far as possible, beyond LLVM's default ``` You can combine multiple `autodiff` values using a comma as separator: ```bash RUSTFLAGS="-Z autodiff=Enable,LooseTypes,PrintPerf" cargo +enzyme build ``` Using `-Zautodiff=Enable` will allow using autodiff and update your normal rustc compilation pipeline: 1. Run your selected compilation pipeline. If you selected a release build, we will disable vectorization and loop unrolling. 2. Differentiate your functions. 3. Run your selected compilation pipeline again on the whole module. This time we do not disable vectorization or loop unrolling.

This section describes the experimental `-Z autodiff` rustc flag (passed via `RUSTFLAGS`) that allows modifying Enzyme's behavior without recompiling rustc. It details debug flags like `PrintTA`, `PrintAA`, `Print`, `PrintPerf`, `PrintModBefore`, `PrintModAfter`, `PrintModFinal`, and `LooseTypes`, which aid in debugging and understanding Enzyme's operations. It also covers benchmark flags such as `NoPostopt`, `RuntimeActivity`, and `Inline` for performance analysis. Multiple `autodiff` values can be combined using commas. Using `-Zautodiff=Enable` enables autodiff, updates the compilation pipeline, and disables vectorization and loop unrolling during the initial compilation, but reenables them after differentiation.