"default" to use flashr default function to initialize factors, currently udv_si.
"nonneg" to implement a non-negative constraint on the factors
subset
indices of the subset of data to use (set to NULL for
all data)
remove_singleton
whether or not factors corresponding to singleton matrices should be removed from output
tag
specific a tag to name the contents in the return objects. You may want to choose different tags for
different paramter combinations. Default is set to init_fn parameter in flashr::flash.
output_model
if specified a filename, the FLASH model will be saved to that file in RDS format.