Build conda package in the new .conda format

[tongyuantongyu]

Conda 4.7 (released more than 4 years ago) introduced the new .conda package format, which uses zstd instead of bzip2 as compression algorithm.

Thanks to zstd, the new package format is both smaller and decompresses faster. conda-forge channel has been releasing packages in .conda format for quite long.

To instruct conda-build to produce packages in .conda format, one can adding these lines to .condarc:

conda_build:
  pkg_format: 2
  zstd_compression_level: 19

or in command line:

conda config --set conda_build.pkg_format 2
conda config --set conda_build.zstd_compression_level 19

And the conda-package-handling package provides cph command to convert packages from old .tar.bz2 format into .conda format:

cph transmute pytorch-2.0.1-py3.11_cuda11.8_cudnn8.7.0_0.tar.bz2 .conda --zstd-compression-threads=-1

As two examples, using the command above, package size of the latest linux64 build pytorch-2.0.1-py3.11_cuda11.8_cudnn8.7.0_0 reduced from 1444MB to 992MB (-31%) and latest win64 build pytorch-2.0.1-py3.10_cuda11.8_cudnn8_0 reduced from 1395MB to 941MB (-33%).

Given that .conda format can significantly speed up installation (during both downloading and unpacking), and is already being widely used, I think it's worth switching to it?