Add 6digit: k=6 addition (5,000,000 train + 5,000 test examples)

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

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:c5c92e94cc8722f3ca04440b160780f7e749d06e6189ccd54a10fac1384d4e1a
+size 104566

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

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:1530970c228b19e24773e7f9ed7cc8b5281edcb12173a7e89dd2589dc821bf12
+size 96019811