test_de failing with on_disk models

Hello Constantin,

Thank you for this code, I've been using and learning more about it for several months. I'm working with some large data sets and using glmGamPoi to perform modeling and hypothesis testing.

I've been able to create full models from HDF5Array input data sets and on_disk = TRUE. I did need to adjust the chunk size params to get this to occur in finite time. However, the on-disk version of the model output cannot be queried with test_de. Performing such test gives the error:

Error in combine_size_factors_and_offset(offset, size_factors, Y, verbose = verbose) :
length(offset) == 1 || length(offset) == n_samples is not TRUE

(seems to be coming from estimate_size_factors.R)

If I run the same model with a realized input matrix and on_disk = FALSE, everything works as expected. I wonder if test_de can't determine the right dimensions or related parameters for on_disk models and is instead throwing this error?

It could be a general bug, it could be a problem that only shows up when one alters default H5 temp file paths, etc? But I've looked, and I think the on-disk model objects have hard-coded locations for data contained within them.... Hmm.

Attached is the log file (docx). You can get a full tgz of the code and input data at the link below. I've tried to make this an MRE, but even very truncated the singlecellexperiment file is a little big.

Thanks, Nathan Siemers
[mre.glmgampoi.bug.docx](https://github.com/const-ae/glmGamPoi/files/8383720/mre.glmgampoi.bug.docx)

tgz file: https://drive.google.com/file/d/1fYN4F7TtFpwgC6yqYZDP_rDHBiV4XU5H/view?usp=sharing