'StandardModel' object has no attribute '_inference_trainer'

[RasmusOrsoe]

Describe the bug
When you load in a saved state dict to a freshly built model, you cannot do inference with that model because the inference code itself relies on Trainer which is only available if model.fit() has been called.

To Reproduce
Steps to reproduce the behavior:

1. Run any training example
2. Replace the code in the example that trains the model, with the line

model.load_state_dict(state_dict_path)

where state_dict_path is the path of a saved state dict.
5. Proceed to run the part of the example code that does inference with this model.
6. See error

Expected behavior
A clear and concise description of what you expected to happen.

Full traceback
Please include the full error message to allow for debugging

AttributeError                            Traceback (most recent call last)
/tmp/ipykernel_931/847469493.py in <module>
      1 # Inference
----> 2 results = inference(model, config)
      3 
      4 # Read predictions from disk (fastest)
      5 #results = pd.read_csv('')

/tmp/ipykernel_931/157537399.py in inference(model, config)
    156         test_dataloader,
    157         prediction_columns=model.prediction_columns,
--> 158         additional_attributes=model.additional_attributes,
    159     )
    160     # Save predictions and model to file

/kaggle/working/software/graphnet/src/graphnet/models/model.py in predict_as_dataframe(self, dataloader, prediction_columns, node_level, additional_attributes, index_column)
    138             )
    139 
--> 140         predictions_torch = self.predict(dataloader)
    141         predictions = (
    142             torch.cat(predictions_torch, dim=1).detach().cpu().numpy()

/kaggle/working/software/graphnet/src/graphnet/models/standard_model.py in predict(self, dataloader)
    166         """Return predictions for `dataloader`."""
    167         self.inference()
--> 168         return super().predict(dataloader)

/kaggle/working/software/graphnet/src/graphnet/models/model.py in predict(self, dataloader)
     93         self.train(mode=False)
     94 
---> 95         predictions_list = self._inference_trainer.predict(self, dataloader)
     96         assert len(predictions_list), "Got no predictions"
     97 

/opt/conda/lib/python3.7/site-packages/torch/nn/modules/module.py in __getattr__(self, name)
   1184                 return modules[name]
   1185         raise AttributeError("'{}' object has no attribute '{}'".format(
-> 1186             type(self).__name__, name))
   1187 
   1188     def __setattr__(self, name: str, value: Union[Tensor, 'Module']) -> None:

AttributeError: 'StandardModel' object has no attribute '_inference_trainer'

Additional context
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