allow custom trainer_cls to be defined as a module reference in the YAML (#3024) [skip ci]

* allow custom trainer_cls to be defined as a module reference in the YAML

* address PR feedback and add test

* add tests

tests/utils/test_import_helper.py

@@ -0,0 +1,37 @@
+"""
+test cases for axolotl.utils.import_helper
+"""
+
+import pytest
+
+from axolotl.utils.import_helper import get_cls_from_module_str
+
+
+def test_get_cls_from_module_str():
+    cls = get_cls_from_module_str("axolotl.core.trainers.base.AxolotlTrainer")
+    assert cls.__name__ == "AxolotlTrainer"
+
+
+def test_get_cls_from_module_str_empty_string():
+    with pytest.raises(ValueError, match="module_str must be a non-empty string"):
+        get_cls_from_module_str("")
+
+
+def test_get_cls_from_module_str_whitespace_only():
+    with pytest.raises(ValueError, match="module_str must be a non-empty string"):
+        get_cls_from_module_str("   ")
+
+
+def test_get_cls_from_module_str_invalid_format():
+    with pytest.raises(ValueError, match="Invalid module string format"):
+        get_cls_from_module_str("single_part")
+
+
+def test_get_cls_from_module_str_nonexistent_module():
+    with pytest.raises(ImportError, match="Failed to import module"):
+        get_cls_from_module_str("nonexistent.module.Class")
+
+
+def test_get_cls_from_module_str_nonexistent_class():
+    with pytest.raises(AttributeError, match="Class 'NonExistentClass' not found"):
+        get_cls_from_module_str("axolotl.core.trainers.base.NonExistentClass")