Fix compression of first & last layers of bitnet to int8 (#3738)

### Changes

Added u2 type support in NNCF graph

### Reason for changes

int8 compression of first/last layer of bitnet fails with error:
```
    raise NotImplementedError(msg)
NotImplementedError: NNCF is not yet supported OpenVINO data type: u2.
```

### Related tickets

CVS-176501

### Tests

manual, since need optimum from main branch and openvino==2025.4
and OV doesn't expose `ov.Type.u2` in Python API (requested for 2026.0
release)

```shell
$ pip install git+https://github.com/huggingface/optimum-intel.git 
$ pip install --pre openvino==2025.4.0rc2 openvino-tokenizers==2025.4.0.0rc2 openvino-genai==2025.4.0.0rc2 --extra-index-url https://storage.openvinotoolkit.org/simple/wheels/nightly 
$ optimum-cli export openvino -m optimum-intel-internal-testing/tiny-random-bitnet --weight-format int8 out_dir --task=text-generation-with-past
$ optimum-cli export openvino -m microsoft/bitnet-b1.58-2B-4T --weight-format int8 out_dir --task=text-generation-with-past

```
<img width="1024" height="143" alt="image"
src="https://github.com/user-attachments/assets/ffd2b816-88d3-4a35-bc61-5048f7cc4f9b"
/>

Author: ljaljushkin

src/nncf/openvino/graph/nncf_graph_builder.py

@@ -58,6 +58,7 @@ def convert_to_nncf_dtype(ov_type: ov.Type) -> Dtype:
             "i32": "int",
             "i64": "int",
             "u1": "int",
+            "u2": "int",
             "u4": "int",
             "u8": "int",
             "u16": "int",