Improve the constness of code #1

[lancerts]

https://github.com/karpathy/llm.c/issues/147 Code changes are tested by execution

Scanned through the following files and touched 3 files (2 files are OK wrt constness)

1. classifier_fused.cu
2. crossentropy_foward.cu
3. crossentropy_softmax_backward.cu
4. gelu_forward
5. layernorm_backward

Refactor for other files in subsequent PRs