Address PR comments

crates/models/gpt2/src/lib.rs

@@ -31,6 +31,8 @@ pub struct Gpt2 {
     // weighted positional encodings
     wpe: Tensor,
     // language model head
+    //
+    // Optional: if not present, the `wte` tensor is used instead.
     lm_head: Option<Tensor>,
 
     // weights for the model
@@ -59,6 +61,9 @@ impl KnownModel for Gpt2 {
         let ln_f_b = tl.load("model/ln_f/b")?;
         let wte = tl.load("model/wte")?;
         let wpe = tl.load("model/wpe")?;
+
+        // GPT-2's language model head is optional; if it is not present,
+        // the `wte` tensor is used instead.
         let lm_head = tl.load("model/lm_head").ok();
 
         let mut layers = Vec::new();