The configuration of 3.0.2 and 3.1.0 is not compatible
The configuration of **3.0.2** and **3.1.0** is not compatible.

For example, in https://github.com/huggingface/transformers/blob/master/src/transformers/modeling_bert.py#L388, `config.chunk_size_feed_forward` should change to `config.get('chunk_size_feed_forward', 0)`, because there is not `chunk_size_feed_forward` config in former version.

```
class BertLayer(nn.Module):
    def __init__(self, config):
        super().__init__()
        # self.chunk_size_feed_forward = config.chunk_size_feed_forward
        self.chunk_size_feed_forward = config.get('chunk_size_feed_forward', 0)
        self.seq_len_dim = 1
        self.attention = BertAttention(config)
        self.is_decoder = config.is_decoder
        self.add_cross_attention = config.add_cross_attention
        if self.add_cross_attention:
            assert self.is_decoder, f"{self} should be used as a decoder model if cross attention is added"
            self.crossattention = BertAttention(config)
        self.intermediate = BertIntermediate(config)
        self.output = BertOutput(config)
```
issue #6950 is also this problem.<|||||>I see what you mean! From `3.1.0` onwards every configuration has a `config.chunk_size_feed_forward` parameter. So as far as I can see whenever a config is loaded (whether via `.from_pretrained()` or with `BertConfig(....)`, this parameter is part of the config...). Can you give me an example where this would not be the case? <|||||>@patrickvonplaten Thanks for your replay.

Yes, I load the pretrained model with my own code, which cause this problem.
For some users who only use part of structures/classes/others in Transfomers, is it necessary to have certain compatibility?<|||||>> I see what you mean! From `3.1.0` onwards every configuration has a `config.chunk_size_feed_forward` parameter. So as far as I can see whenever a config is loaded (whether via `.from_pretrained()` or with `BertConfig(....)`, this parameter is part of the config...). Can you give me an example where this would not be the case?

I saw the `chunk_size_feed_forward` in BertConfig() after opened this issue.🤪<|||||>@patrickvonplaten 
In the future, I will make more normative to call modules.
Thanks for your replay.
I close this issue.