[FEA] Support fp16 input type in stats::mean()

[tfeher]

Support fp16 operations for raft::statt::mean()

Describe the solution you'd like

fp16 support where the intermediate results are accumulated in fp32, to avoid potential overflow.

Additional context

This issue came up in rapidsai/cuvs#716, and resulted in the following workaround

https://github.com/rapidsai/cuvs/pull/716/files#diff-15fe794d70734cd604f7fda31ee8c2240839939c1c811df6cceb71752a0c35c8R242