add FastLinear import (#750)

# What does this PR do?

Fixes #749 

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Co-authored-by: p_spozzhang <p_spozzhang@tencent.com>

server/text_generation_server/models/custom_modeling/opt_modeling.py

@@ -28,6 +28,7 @@ from transformers.modeling_outputs import (
 from transformers.modeling_utils import PreTrainedModel
 from transformers import OPTConfig
 from text_generation_server.utils.layers import (
+    FastLinear,
     TensorParallelColumnLinear,
     TensorParallelEmbedding,
     TensorParallelRowLinear,