v0.29.0

In this release:

• New metal backend for apple systems. This is now the default backend for macos builds.
• New onnx-dml backend to use DirectML under windows, has better net compatibility than dx12 and is faster than opencl. See the README for use instructions, a separate download of the DirectML dll is required.
• Full attention policy support in cuda, cudnn, metal, onnx, blas, dnnl, and eigen backends.
• Partial attention policy support in onednn backend (good enough for T79).
• Non multigather (legacy) search code and --multigather option are removed.
• Now the onnx backends can use fp16 when running with a network file (not with .onnx model files). This is the default for onnx-cuda and onnx-dml, can be switched on or off with by setting the fp16 backend option to true or false respectively.
• The onednn package comes with the latest dnnl compiled to allow running on an intel gpu by adding gpu=0 to the backend options.
• The default net is now 791556 for most backends except opencl and dx12 that get 753723 (as they lack attention policy support).
• Support for using pgn book with long lines in training: selfplay can start at a random point in the book.
• New "simple" time manager.
• Support for double Fischer random chess (dfrc).
• Added TC-dependent output to the backendbench assistant.
• Starting with this version, the check backend compares policy for valid moves after softmax.
• The onnx backend now allows selecting gpu to use.
• Improved error messages for unsupported network files.
• Some assorted fixes and code cleanups.