`Recorder` does not work with models with non-`Array` inputs.

[lorenzoh]

Since Recorder tries to count the number of samples in a batch separately from those in a batch, it fails to get the batch size if the model input xs is not an array.

Possible solutions:

1. remove the sample-counting
2. add batchsize detection cases for other input types like Tuples and Dicts

I prefer 1. since the feature is not currently used, and it is not possible to exhaustively implement extra cases for batch size detection. Recorder should focus on counting the steps.

[lorenzoh]

Fixed by #81