[Core] Subclass ModelRunner to support cross-attention & encoder sequences (towards eventual encoder/decoder model support) (#4942)

Co-authored-by: Andrew Feldman <afeld2012@gmail.com>
Co-authored-by: Nick Hill <nickhill@us.ibm.com>

tests/kernels/test_attention_selector.py

@@ -3,9 +3,9 @@ from unittest.mock import patch
 import pytest
 import torch
 
-from tests.kernels.utils import (STR_FLASH_ATTN_VAL, STR_INVALID_VAL,
-                                 override_backend_env_variable)
+from tests.kernels.utils import override_backend_env_variable
 from vllm.attention.selector import which_attn_to_use
+from vllm.utils import STR_FLASH_ATTN_VAL, STR_INVALID_VAL
 
 
 @pytest.mark.parametrize(