Defer boto3 import and raise actionable ImportError for s3:// loads (#489)

*Issue #, if available:*

*Description of changes:*
- `boto3` lives in `[project.optional-dependencies].extras`, but
`src/chronos/boto_utils.py` imported it (and `botocore`) at module top.
A user who installs the package with `pip install chronos-forecasting`
(no extras) and then calls `from_pretrained("s3://...")` currently hits
a bare `ModuleNotFoundError: No module named 'boto3'` with no hint about
the `extras` group.
- Defer `boto3` / `botocore` imports in `src/chronos/boto_utils.py` into
the single function that actually uses them
(`download_model_files_from_s3`) and move the `boto3.Session` annotation
behind `from __future__ import annotations` + a `TYPE_CHECKING` block,
so importing the package no longer fails when `boto3` is absent.
- In `src/chronos/base.py` (inside
`BaseChronosPipeline.from_pretrained`, around line 349) wrap the
`cache_model_from_s3(...)` call in `try/except ImportError` and re-raise
with a message that tells the user to run `pip install
'chronos-forecasting[extras]'`. HuggingFace Hub and local-path loads are
unaffected.
- Verified locally in a fresh venv (no `[extras]`): `from chronos import
BaseChronosPipeline` succeeds,
`BaseChronosPipeline.from_pretrained("s3://...")` now raises the
actionable `ImportError` instead of the bare `ModuleNotFoundError`.


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choice.

Co-authored-by: Abdul Fatir <Abdulfatirs@gmail.com>

src/chronos/base.py

@@ -348,9 +348,16 @@ class BaseChronosPipeline(metaclass=PipelineRegistry):
         if str(pretrained_model_name_or_path).startswith("s3://"):
             from .boto_utils import cache_model_from_s3
 
-            local_model_path = cache_model_from_s3(
-                str(pretrained_model_name_or_path), force_download=force_s3_download
-            )
+            try:
+                local_model_path = cache_model_from_s3(
+                    str(pretrained_model_name_or_path), force_download=force_s3_download
+                )
+            except ImportError as e:
+                raise ImportError(
+                    "Loading models from s3:// URIs requires boto3. "
+                    "Install the optional dependencies with: "
+                    "pip install 'chronos-forecasting[extras]'"
+                ) from e
             return cls.from_pretrained(local_model_path, *model_args, **kwargs)
 
         from transformers import AutoConfig

src/chronos/boto_utils.py

@@ -3,17 +3,19 @@
 
 # Authors: Abdul Fatir Ansari <ansarnd@amazon.com>
 
+from __future__ import annotations
+
 import logging
 import os
 import re
 import warnings
 from pathlib import Path
+from typing import TYPE_CHECKING
 
-import boto3
 import requests  # type: ignore
-from botocore import UNSIGNED
-from botocore.client import Config
-from botocore.exceptions import ClientError, NoCredentialsError
+
+if TYPE_CHECKING:
+    import boto3
 
 logger = logging.getLogger(__name__)
 
@@ -57,6 +59,11 @@ def download_model_files_from_s3(
     force_download: bool = False,
     boto3_session: boto3.Session | None = None,
 ) -> None:
+    import boto3
+    from botocore import UNSIGNED
+    from botocore.client import Config
+    from botocore.exceptions import ClientError, NoCredentialsError
+
     boto3_session = boto3_session or boto3.Session()
     s3_client = boto3_session.client("s3")