Bugfix for unfreeze layers

Another missed occurrence of "vit" rather than "vt". Also fixed to look for first two characters in name rather than anywhere in name, as per the new standard across the code.

I think I've got all of those cases now!

src/lib_model.py

@@ -129,15 +129,15 @@ def build_classifier(config, num_classes, df_size, img_size):
 def find_unfreeze_points(model, mname, blocks_to_unfreeze):
     block_starts = []
     print("\nModel Name =", mname)
-    if 'cn' in mname.lower():
+    if 'cn' in mname[:2].lower():
         for layer in model.layers:
             if 'stages' in layer.name and layer.name.endswith('_downsample_1_conv2d'):
                 block_starts.append(layer.name)
-    elif 'en' in mname.lower():
+    elif 'en' in mname[:2].lower():
         for layer in model.layers:
             if 'block' in layer.name and layer.name.endswith('_0_conv_pw_conv2d'):
                 block_starts.append(layer.name)
-    elif 'vit' in mname.lower():
+    elif 'vt' in mname[:2].lower():
         for layer in model.layers:
             if 'blocks' in layer.name and layer.name.endswith('_attn'):
                 block_starts.append(layer.name)