Expose OpBuilder as onnxscript.OpBuilder

[gramalingam]

Add OpBuilder to all and import it from _internal.builder in the top-level onnxscript package.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
13061	1	13060	2257

View the top 1 failed test(s) by shortest run time

onnxscript.rewriter.ort_fusions.rms_normalization_test.TestRmsNormalization::test_smollm

Stack Traces | 16.5s run time

onnxscript\rewriter\ort_fusions\rms_normalization_test.py:23: in test_smollm
    new_outputs = ort_run("optimized", model, inputs)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
onnxscript\rewriter\ort_fusions\_test_utils.py:19: in ort_run
    model_proto.SerializeToString(), options, providers=providers
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E   google.protobuf.message.EncodeError: Failed to serialize proto

View the full list of 1 ❄️ flaky test(s)

tests.function_libs.torch_lib.ops_test.TestOutputConsistencyFullGraphCPU::test_output_match_opinfo__logsumexp_cpu_float16

Flake rate in main: 14.58% (Passed 252 times, Failed 43 times)

Stack Traces | 0.165s run time

.../function_libs/torch_lib/ops_test.py:243: in run_test_output_match
    torch.testing.assert_close(
E   AssertionError: Tensor-likes are not close!
E   
E   Mismatched elements: 1 / 5 (20.0%)
E   Greatest absolute difference: 2.288818359375e-05 at index (1,) (up to 1e-05 allowed)
E   Greatest relative difference: 0.0022869110107421875 at index (1,) (up to 0.001 allowed)

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