Add 6MT-3digit: exhaustive 3-digit addition (1,000,000 examples, 6,499,500 tokens)

6MT-3digit/test-00000-of-00001.parquet

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:bec9b70075658e87b5c06cf8d41ee660bcd88fd492ebc9dd47e64c096aa62966
+size 1222680

6MT-3digit/train-00000-of-00001.parquet

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:71204e261eeac5afe7336cda77c78e42ec7914d29a04db735c88260f1f1d66e3
+size 10131045