Relax Transformers modeling backend MoE experts check (#28952)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/models/supported_models.md

@@ -79,7 +79,9 @@ To make your model compatible with the Transformers modeling backend, it needs:
         1. Add `is_causal = False` to `MyAttention`.
     - If your model is mixture-of-experts (MoE):
         1. Your sparse MoE block must have an attribute called `experts`.
-        2. The class of `experts` (`MyExperts`) must inherit from `nn.ModuleList`.
+        2. The class of `experts` (`MyExperts`) must either:
+            - Inherit from `nn.ModuleList` (naive).
+            - Or contain all 3D `nn.Parameters` (packed).
         3. `MyExperts.forward` must accept `hidden_states`, `top_k_index`, `top_k_weights`.
 2. `MyAttention` must use `ALL_ATTENTION_FUNCTIONS` to call attention.
 3. `MyModel` must contain `_supports_attention_backend = True`.