v0.29.0-rc0

In this release:

• Initial support for attention policy, only cuda backend and partially in
  blas/dnnl/eigen (good enough for T79).
• Non multigather (legacy) search code and --multigather option are removed.
• 15b default net is now 753723.
• The onnx backend now allows selecting gpu to use.
• Improved error messages for unsupported network files.
• Some assorted fixes.