More consistent variable naming

[vwxyzjn]

Hi @karpathy this is such an awesome repo. I was implementing it from scratch using JAX and learned quite a bit :)

This PR proposes a more consistent naming convention.

• n_embd -> embd_dim, since we are really splitting the embd_dim into separate heads, which have their own head_dim
• C // self.n_head -> head_dim to give a better name
• hs -> hd, to unify naming from "head size" to "head dim", which connects nicely with embd_dm
• att -> attn for consistency
• tok_emb -> tok_embd for consistency with using embd as an abbreviation for embedding
• pos_emb -> pos_embd for consistency with using embd as an abbreviation for embedding