[BugFix] Fix import error in nsa examples when `fla.__version__ >=0.2.1` (#579)

* Update FLA import path for `prepare_token_indices`

* Update FLA import path for `prepare_token_indices`

* Compare versions via packaging.version.parse

examples/deepseek_nsa/example_tilelang_nsa_bwd.py

 # ruff: noqa
 import torch
 from typing import Optional, Union
+from packaging.version import parse
 
 import torch
 import triton
 
-from fla.ops.common.utils import prepare_token_indices
+import fla
+if parse(fla.__version__) < parse("0.2.1"):
+    from fla.ops.common.utils import prepare_token_indices
+else:
+    from fla.ops.utils import prepare_token_indices
 from fla.utils import autocast_custom_bwd, autocast_custom_fwd, contiguous
 from reference import naive_nsa
 from einops import rearrange

examples/deepseek_nsa/example_tilelang_nsa_fwd_varlen.py

 # ruff: noqa
 import torch
 from typing import Optional, Union
+from packaging.version import parse
+
 import tilelang
 from tilelang import language as T
 import tilelang.testing
 
-from fla.ops.common.utils import prepare_token_indices
+import fla
+if parse(fla.__version__) < parse("0.2.1"):
+    from fla.ops.common.utils import prepare_token_indices
+else:
+    from fla.ops.utils import prepare_token_indices
 from reference import naive_nsa
 from einops import rearrange
 

examples/deepseek_nsa/example_triton_nsa_bwd.py

 # ruff: noqa
 import torch
 from typing import Optional, Union
+from packaging.version import parse
 
 import torch
 import triton
 import triton.language as tl
 
-from fla.ops.common.utils import prepare_token_indices
+import fla
+if parse(fla.__version__) < parse("0.2.1"):
+    from fla.ops.common.utils import prepare_token_indices
+else:
+    from fla.ops.utils import prepare_token_indices
 from fla.utils import autocast_custom_bwd, autocast_custom_fwd, contiguous
 from reference import naive_nsa
 from einops import rearrange

examples/deepseek_nsa/example_triton_nsa_fwd.py

 # ruff: noqa
 import torch
 from typing import Optional, Union
+from packaging.version import parse
 
 import torch
 import triton
 import triton.language as tl
 
-from fla.ops.common.utils import prepare_token_indices
+import fla
+if parse(fla.__version__) < parse("0.2.1"):
+    from fla.ops.common.utils import prepare_token_indices
+else:
+    from fla.ops.utils import prepare_token_indices
 from fla.utils import autocast_custom_fwd, contiguous
 from reference import naive_nsa
 from einops import rearrange

examples/deepseek_nsa/example_triton_nsa_fwd_varlen.py

 # ruff: noqa
 import torch
 from typing import Optional, Union
+from packaging.version import parse
 
 import torch
 import triton
 import triton.language as tl
 
-from fla.ops.common.utils import prepare_token_indices
+import fla
+if parse(fla.__version__) < parse("0.2.1"):
+    from fla.ops.common.utils import prepare_token_indices
+else:
+    from fla.ops.utils import prepare_token_indices
 from fla.utils import autocast_custom_fwd, contiguous
 from reference import naive_nsa
 from einops import rearrange