Adafactor lacking min_dim_size_to_factor
The Adafactor implementation in mesh_tensorflow has a `min_dim_size_to_factor` flag where tensors with size smaller than this will not be factored.
https://github.com/tensorflow/mesh/blob/57ed4018e6a173952501b074daabad32b6449f3d/mesh_tensorflow/optimize.py#L220

However, this flag is lacking in HF's version. And there are indeed parameters that are smaller than this, for example, T5's `relative_attention_bias`.

First of all, this causes an implementation mismatch. Secondly, this prevents the loading of pretrained optimizer states into HF.
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