Set litdata < 0.2.6 (#1400)

pyproject.toml

@@ -37,7 +37,7 @@ all = [
     "sentencepiece>=0.2.0",      # llama-based models
     "tokenizers>=0.15.2",        # pythia, falcon, redpajama
     "requests>=2.31.0",          # litgpt.data
-    "litdata>=0.2.2",            # litgpt.data
+    "litdata>=0.2.2,<0.2.6",     # litgpt.data
     "litserve>=0.1.0",           # litgpt.deploy
     "zstandard>=0.22.0",         # litgpt.data.prepare_slimpajama.py
     "pandas>=1.9.0",             # litgpt.data.prepare_starcoder.py