Target factory for batched dynamic-within-static branching for data frames

[ropensci]

Target factory for batched dynamic-within-static branching for data frames

[wlandau]

For (2), I am thinking of something like the following. stantargets and jagstargets already use this idea, e.g. for tar_stan_mcmc_rep_summary().

For (3), there are two stages of row-mapping.

[wlandau]

There are actually 3 different kinds of target factory that would help with future targetopia packages:

1. Batched dynamic branching over rows of a data frame, where each row of the data frame is a set of arguments of the command.
2. Unbatched static branching over the rows of a data frame, followed by dynamic branching to run independent replicates for each row of that data frame. Sort of like tar_rep() inside tar_map().
3. Unbatched static branching over the rows of a data frame, where each static branch produces a new data frame. Then another wave of downstream targets dynamically maps (in batches) over the rows of each new data frame, taking row groups as batches and columns as arguments.

[wlandau]

On second thought, (1) is actually covered by tar_group_count() and tar_group_size().

[wlandau]

I am calling (2) tar_map_rep(). For (3), I will implement tar_map2_count() and tar_map2_size(), each with a different method of partitioning downstream rows into batches.