Repository: huggingface/transformers
Pull request: Fix errors when use verl to train GLM4.1v model

* Support glm4v load from AutoModelForVision2Seq
* Set glm4v model's _checkpoint_conversion_mapping attribute from None to empty dict {}

# What does this PR do?
When use [verl](https://github.com/volcengine/verl) to train GLM4.1v model with GRPO, there are several small errors.
Here is how to fix them:
- support glm4v load using AutoModelForVision2Seq
- verl treat _checkpoint_conversion_mapping as a dict. But right now is None, which will abort the program. I also found that almost every model which don't need checkpoint convert have a empty dict.

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Changed files:
filename: src/transformers/models/glm4v/modeling_glm4v.py
patch: @@ -949,7 +949,7 @@ def forward(
 @auto_docstring
 class Glm4vModel(Glm4vPreTrainedModel):
     base_model_prefix = ""
-    _checkpoint_conversion_mapping = None
+    _checkpoint_conversion_mapping = {}
     config_class = Glm4vConfig
     _no_split_modules = ["Glm4vTextDecoderLayer", "Glm4vVisionBlock"]
 
@@ -1382,7 +1382,7 @@ class Glm4vCausalLMOutputWithPast(ModelOutput):
 
 
 class Glm4vForConditionalGeneration(Glm4vPreTrainedModel, GenerationMixin):
-    _checkpoint_conversion_mapping = None
+    _checkpoint_conversion_mapping = {}
     _tied_weights_keys = ["lm_head.weight"]
 
     def __init__(self, config):

filename: src/transformers/models/glm4v/modular_glm4v.py
patch: @@ -1001,7 +1001,7 @@ def forward(
 
 
 class Glm4vModel(Qwen2_5_VLModel):
-    _checkpoint_conversion_mapping = None
+    _checkpoint_conversion_mapping = {}
     _no_split_modules = ["Glm4vTextDecoderLayer", "Glm4vVisionBlock"]
 
     def __init__(self, config):
@@ -1356,7 +1356,7 @@ class Glm4vCausalLMOutputWithPast(Qwen2_5_VLCausalLMOutputWithPast):
 
 
 class Glm4vForConditionalGeneration(Qwen2_5_VLForConditionalGeneration):
-    _checkpoint_conversion_mapping = None
+    _checkpoint_conversion_mapping = {}
 
     def forward(
         self,