fix(train): bound pydantic to <3.0.0 for cross-package consistency (#6363)

sagemaker-train declared pydantic>=2.0.0 with no upper bound, while
sagemaker-core declares pydantic>=2.0.0,<3.0.0. Installing sagemaker-train
on its own could therefore resolve a different pydantic major than the
rest of the SDK allows. Align the bound so all sub-packages agree on the
pydantic major, letting pip resolve a single consistent pydantic (and its
matching, pydantic-managed pydantic-core).

Note: pydantic-core is intentionally not pinned; pydantic pins its own
exact pydantic-core, and pinning it here would fight that mechanism.

Relates-to #5652

Co-authored-by: Roja Reddy Sareddy <rsareddy@amazon.com>

Author: rsareddy0329

sagemaker-train/pyproject.toml

@@ -44,7 +44,7 @@ dependencies = [
     "mlflow>=3.0.0,<4.0.0",
     "nest_asyncio>=1.5.0",
     "requests>=2.28.0",
-    "pydantic>=2.0.0",
+    "pydantic>=2.0.0,<3.0.0",
 ]
 
 [project.urls]