[Refactor] Relative imports wherever we can (#21880)

* initial commit

* update

* second batch

* style

* fix imports

* fix relative import on pipeline

src/transformers/pipelines/pt_utils.py

@@ -2,7 +2,7 @@ import numpy as np
 import torch
 from torch.utils.data import Dataset, IterableDataset
 
-from transformers.utils.generic import ModelOutput
+from ..utils.generic import ModelOutput
 
 
 class PipelineDataset(Dataset):

src/transformers/pipelines/text_generation.py

@@ -1,8 +1,7 @@
 import enum
 import warnings
 
-from transformers import MODEL_FOR_CAUSAL_LM_MAPPING, TF_MODEL_FOR_CAUSAL_LM_MAPPING
-
+from .. import MODEL_FOR_CAUSAL_LM_MAPPING, TF_MODEL_FOR_CAUSAL_LM_MAPPING
 from ..utils import add_end_docstrings, is_tf_available
 from .base import PIPELINE_INIT_ARGS, Pipeline