[BUG] Stats kernels invalid access

[tfeher]

Describe the bug

Similar to https://github.com/rapidsai/raft/pull/2183, the mean and stdev kernels have also potential out of bounds access

https://github.com/rapidsai/raft/blob/branch-24.04/cpp/include/raft/stats/detail/stddev.cuh#L48 https://github.com/rapidsai/raft/blob/branch-24.04/cpp/include/raft/stats/detail/mean.cuh#L46

Additionally the minmax kernel should be also checked, whether it has a similar problem.

Steps/Code to reproduce bug

• Add a similar test that was added in #2183.
• Run compute-sanitizer gtests/STATS_TEST. Without compute-sanitizer the test could silently pass.

Additional context

• We have to add tests that cover edge cases so that such errors are not occuring