Mixed-precision training

The most important new feature of this release is mixed-precision training 🎉. This speeds up training and lowers memory use on GPUs with Tensor Cores. Mixed-precision training can be enabled using the --mixed-precision option of sticker2 finetune and sticker2 distill.

Other notable changes:

• Use fast AVX2 kernels on AMD Zen CPUs, without setting any special environment variables.
• Update the sentencepiece crate dependency to 0.4. This version compiles the sentencepiece library statically if it is not available, removing the dependency on an external sentencepiece build.
• The TensorBoard summary writer support that was added in 0.4.2 is now feature-gated (tensorboard). This makes it possible to compile sticker2 without TensorBoard support for quicker compiles and smaller binaries.