Off-by-one error in clean! method

Bug description

When a model is constructed with batch_size=0 it passes the clean! method without warning and batch_size remains at value 0. However the fit! method attempts to collate the data into batches which is not possible due to the Julia partition method (please find the method below) called in collate throwing an error when attempting to set n as 0.

function partition(c, n::Integer)
    n < 1 && throw(ArgumentError("cannot create partitions of length $n"))
    return PartitionIterator(c, Int(n))
end

I am not entirely certain if 0 is meant to stand for no batching or if un-batched training is not an intended feature, but currently I am assuming it is the latter due to the way training is done, with un-batched training basically not being possible. Therefore I assume this is an off-by-one error and have made the clean! method reject batch_size=0 along with everything below that.

PR Checklist

[x] Tests are added

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.34 :tada:

Comparison is base (bf410e1) 92.92% compared to head (97897ae) 93.26%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #227 +/- ## ========================================== + Coverage 92.92% 93.26% +0.34% ========================================== Files 11 12 +1 Lines 311 312 +1 ========================================== + Hits 289 291 +2 + Misses 22 21 -1 ``` | [Impacted Files](https://app.codecov.io/gh/FluxML/MLJFlux.jl/pull/227?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Flux) | Coverage Δ | | |---|---|---| | [src/mlj\_model\_interface.jl](https://app.codecov.io/gh/FluxML/MLJFlux.jl/pull/227?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Flux#diff-c3JjL21sal9tb2RlbF9pbnRlcmZhY2Uuamw=) | `94.20% <100.00%> (ø)` | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/FluxML/MLJFlux.jl/pull/227/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Flux)

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