Support pre-packing weights after model optimization

This reduces inference time for matmul operations at the cost of higher memory
usage.

 - Add methods to `Operator` trait to declare which inputs can
   potentially be pre-packed and to prepack those inputs.

 - Add `Graph::prepack_weights` method to traverse operators and prepack
   inputs whose values are constant nodes.

 - Implement prepacking methods for MatMul and fused MatMul ops

There are some caveats:

 - Non-MatMul operations which use matmuls internally (Conv, ConvTranspose,
   LSTM, GRU etc.) currently don't prepack their weights.

 - MatMul operations which turn out to be matrix-vector (gemv) products don't
   use the prepacked weights. This affects transformer decoders doing
   non-batched generation after the initial prompt encoding step.

src/gemm.rs

@@ -108,6 +108,16 @@ impl<T> PackedBMatrix<T> {
     fn panel_len(&self) -> usize {
         self.panel_width * self.rows
     }
+
+    /// Number of rows in the unpacked matrix.
+    pub fn rows(&self) -> usize {
+        self.rows
+    }
+
+    /// Number of columns in the unpacked matrix.
+    pub fn cols(&self) -> usize {
+        self.cols
+    }
 }
 
 impl<T> ExtractBuffer for PackedBMatrix<T> {