Fix compiler warnings treated as errors in GistEncodeDecode. (#7568)
* Fix compiler warning in GistEncodeDecode.
* Fix other use of member variable.
* Make `compression_type_` const.
* Change floor to floorf in CUDA code.
* Statically cast size_t to int in GIST CUDA kernels
* Add explicit cast to `long` in gist.cc
Co-authored-by: Derek Murray <demurra@microsoft.com>