Avoid integer overflows in EigenMetaKernel indexing

Ben Barsdellrequested to merge
benbarsdell/eigen:fix-metakernel-indexing-overflows into master
  • The current implementation computes size + total_threads, which can overflow and cause CUDA_ERROR_ILLEGAL_ADDRESS when size is close to the maximum representable value.
  • The num_blocks calculation can also overflow due to the implementation of divup().
  • This patch prevents these overflows and allows the kernel to work correctly for the full representable range of tensor sizes.
  • Also adds relevant tests.

cc @nluehr

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