SimpleHDFWriter extra seq lens not correct, not supporting custom seq lens

[albertz]

In SimpleHDFWriter.insert_batch, when using extra to put some other data (despite for the main data stream "data"), the seq lens are currently not handled correctly.

Currently the logic is:

        ...
        for i in range(n_batch):
            ...
                    for key, value in extra.items():
                        assert value.shape[0] == n_batch
                        self._insert_h5_other(key, value[i])

I.e. for some value of shape [B,T,F], if will use the seq len always as long as the max seq len of the current batch. That means:

• The seq len is wrong.
• There is no support for custom seq lens of extra in any case.

(cc @dorian-K @NeoLegends)