fix lint: lowercase variable name, add install hints for missing writers

Signed-off-by: yashnaiduu <yashnaiduu@users.noreply.github.com>

Author: yashnaiduu

monai/data/image_writer.py

@@ -117,7 +117,7 @@ def resolve_writer(ext_name, error_if_not_found=True) -> Sequence:
             avail_writers.append(_writer)
     if not avail_writers and error_if_not_found:
         # map common extensions to their required package
-        _INSTALL_HINTS: dict = {
+        install_hints: dict = {
             "nii": "nibabel",
             "nii.gz": "nibabel",
             "mha": "itk",
@@ -130,7 +130,7 @@ def resolve_writer(ext_name, error_if_not_found=True) -> Sequence:
             "tiff": "pillow",
             "bmp": "pillow",
         }
-        hint = _INSTALL_HINTS.get(fmt)
+        hint = install_hints.get(fmt)
         extra = f" Try installing the required package: pip install {hint}" if hint else ""
         raise OptionalImportError(f"No ImageWriter backend found for '{fmt}'.{extra}")
     writer_tuple = ensure_tuple(avail_writers)