fix member access to packed CUDA struct

[h-vetinari]

The mosaic code uses the CUDA CUpti_ActivityKernel9 struct, which (at least as of CUDA 12.6) has been declared with __attribute__((packed)). This makes it UB to access a reference to a potentially unaligned field, causing GCC 13 to raise an error:

jaxlib/mosaic/gpu/mosaic_gpu_ext.cc: In function 'void jax::cuda::{anonymous}::callback_complete(CUcontext, uint32_t, uint8_t*, size_t, size_t)':
jaxlib/mosaic/gpu/mosaic_gpu_ext.cc:185:37: error: cannot bind packed field 'kernel->CUpti_ActivityKernel9::name' to 'const char*&'
  185 |             std::make_tuple(kernel->name, duration_ms));
      |                             ~~~~~~~~^~~~

Work around this by separating out the member access, which avoids the packed struct reference issue because then it's just a copy of the pointer, not a reference. We needed this to unblock the distribution of jax in conda-forge (where we're currently on GCC 13.3 & CUDA 12.6): conda-forge/jaxlib-feedstock#295

[apaszke]

Thanks!