Flux2TextEncoder.forward: forward return_dict + cache_position via kwargs

Address gemini-code-assist review comment on PR #1434
(#1434).

The previous patch captured return_dict and cache_position in the
forward signature (for backward compat with older transformers
versions where they were positional) but didn't forward them to
super().forward(). For 5.8+ callers explicitly passing these args,
they would be silently dropped.

Now forwarded the same way as output_hidden_states / output_attentions:
via kwargs.setdefault so explicit kwargs in the call still win, and
None values don't pollute **kwargs.

In transformers 5.8 these two are no-ops (return_dict always True;
cache_position computed internally), so forwarding them only matters
for older transformers versions -- but it's the correct behavior either
way, and a cheap fix.

Author: genno-whittlery

diffsynth/models/flux2_text_encoder.py

@@ -55,13 +55,19 @@ def __init__(self):
 
     def forward(self, input_ids = None, pixel_values = None, attention_mask = None, position_ids = None, past_key_values = None, inputs_embeds = None, labels = None, use_cache = None, output_attentions = None, output_hidden_states = None, return_dict = None, cache_position = None, logits_to_keep = 0, image_sizes = None, **kwargs):
         # transformers 5.8 trimmed Mistral3's positional args; pass everything
-        # as kwargs so the call survives across transformers versions. The dropped
-        # output_hidden_states / output_attentions are still honored via
-        # TransformersKwargs -> forward them through **kwargs.
+        # as kwargs so the call survives across transformers versions. Forward
+        # the four removed-from-positional args through **kwargs (transformers
+        # 5.8 honors output_hidden_states / output_attentions via
+        # TransformersKwargs; return_dict / cache_position are no-ops in 5.8
+        # but forwarded so older transformers versions still receive them).
         if output_hidden_states is not None:
             kwargs.setdefault("output_hidden_states", output_hidden_states)
         if output_attentions is not None:
             kwargs.setdefault("output_attentions", output_attentions)
+        if return_dict is not None:
+            kwargs.setdefault("return_dict", return_dict)
+        if cache_position is not None:
+            kwargs.setdefault("cache_position", cache_position)
         return super().forward(
             input_ids=input_ids,
             pixel_values=pixel_values,