[API 2.0] Add transformer apis (#26418)

* Add MultiHeadAttention api.
test=develop

* Add MultiHeadAttention cache type and gen_cache.
test=develop

* Add TransformerEncoderLayer and TransformerEncoder.
test=develop

* Add Transformer decoder apis.
test=develop

* Add Transformer api.
test=develop

* add unittests for transformer api

* add unittests for transformer api

* Fix some bugs in Transformer apis.
test=develop

* add unittests for encoder, decoder and transformer

* clean conflicts infor in code

* clean Chinese comments

* Add TransformerDecoderCell and TransformerBeamSearchDecoder.
test=develop

* Remove TransformerDecoderCell and TransformerBeamSearchDecoder temporarily.
test=develop

* Add import for Transformer apis.
test=develop

* Update usage of weight_attr and Tensor in Transformer api docs.
test=develop

* Update Transformer apis by renaming MultiheadAttention and cal_kv according to comments.
test=develop

* Fix MultiHeadAttention in test_transformer_api.py.
test=develop
Co-authored-by: NLiuChiaChi <709153940@qq.com>

python/paddle/nn/__init__.py

@@ -130,6 +130,12 @@ from .layer.norm import InstanceNorm  #DEFINE_ALIAS
 # from .layer.rnn import RNNCell        #DEFINE_ALIAS
 # from .layer.rnn import GRUCell        #DEFINE_ALIAS
 # from .layer.rnn import LSTMCell        #DEFINE_ALIAS
+from .layer.transformer import MultiHeadAttention
+from .layer.transformer import TransformerEncoderLayer
+from .layer.transformer import TransformerEncoder
+from .layer.transformer import TransformerDecoderLayer
+from .layer.transformer import TransformerDecoder
+from .layer.transformer import Transformer
 from .layer.distance import PairwiseDistance  #DEFINE_ALIAS
 
 from .layer import loss  #DEFINE_ALIAS

python/paddle/nn/layer/__init__.py

@@ -21,6 +21,7 @@ from . import extension
 from . import activation
 from . import norm
 from . import distance
+from . import transformer
 
 from .activation import *
 from .loss import *
@@ -28,6 +29,7 @@ from .conv import *
 from .extension import *
 from .activation import *
 from .norm import *
+from .transformer import *
 # from .activation import PReLU        #DEFINE_ALIAS
 from .activation import ReLU  #DEFINE_ALIAS
 from .activation import LeakyReLU  #DEFINE_ALIAS