use const properly, esp in function signatures

[karpathy]

I wasn't diligent enough with my use of float* vs const float*. The former is used for outputs, the latter for inputs, to all functions. We'd like to refactor the function declarations to follow this paradigm, to be more defensive.

[rosslwheeler]

This one?

int check_tensor(float *a, float b, int n, const char label) { this one generates a couple of warnings

test_gpt2.cu(139): warning #2464-D: conversion from a string literal to "char *" is deprecated test_gpt2.cu(141): warning #2464-D: conversion from a string literal to "char *" is deprecated

There's also these but wasn't sure if there's some incoming changes to use these variables?

test_gpt2.cu(56): warning #177-D: variable "maxT" was declared but never referenced test_gpt2.cu(57): warning #177-D: variable "L" was declared but never referenced test_gpt2.cu(95): warning C4101: 'losses': unreferenced local variable