[Qwen3] Fix weight tying for Qwen3 according to Huggingface configs (#1633)

As titled. Only enable weight tying for smaller model

Author: wwwjn

torchtitan/experiments/qwen3/__init__.py

@@ -40,6 +40,7 @@
         qk_norm=True,
         hidden_dim=3072,
         rope_theta=1000000,
+        enable_weight_tying=True,
     ),
     "1.7B": Qwen3ModelArgs(
         vocab_size=151936,
@@ -52,6 +53,7 @@
         qk_norm=True,
         hidden_dim=6144,
         rope_theta=1000000,
+        enable_weight_tying=True,
     ),
     "4B": Qwen3ModelArgs(
         vocab_size=151936,
@@ -64,6 +66,7 @@
         qk_norm=True,
         hidden_dim=9728,
         rope_theta=1000000,
+        enable_weight_tying=True,
     ),
     "8B": Qwen3ModelArgs(
         vocab_size=151936,