[DISCUSS] Remove Protobuf dependency

[hcho3]

The protobuf dependency was first introduced when cuML began building Treelite as a submodule. Now, Treelite has an efficient binary serializer (#2263) so Protobuf is no longer necessary for serializing trees. There is now exactly one place in cuML where Protobuf is used:

cuml/python/cuml/fil/fil.pyx

Lines 137 to 142 in 5d6377b

	elif model_type == "protobuf":
	# XXX Not tested
	res = TreeliteLoadProtobufModel(filename_bytes, &handle)
	if res < 0:
	err = TreeliteGetLastError()
	raise RuntimeError("Failed to load %s (%s)" % (filename, err))

which, as the comment suggests, is untested.

My humble opinion is that the feature is small enough to not warrant the introduction of a dependency like Protobuf. I open this pull request to start a discussion. As in #2528, some users want to build cuML without using the Conda package management system, and the Protobuf dependency makes their lives harder.

I will file a separate pull request to document steps for building Treelite manually without Conda.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[dantegd]

I fully agree and support this change! Thanks @hcho3