don't train if eval split is too small (#873)

* allow zero len dataset

* better handling and warning of small eval splits

* raise error if eval split is too small

* don't mess with calculating total num steps in distributed context

* fix eval_sample_packing training args logic

src/axolotl/utils/samplers/multipack.py

@@ -182,7 +182,7 @@ class MultipackBatchSampler(BatchSampler):
 
         # shave off 1% + 1 for dealing with variance in packing from random sampler to sampler
         return max(
-            1,
+            0,
             (
                 world_size
                 * math.floor(