* Add `paddle.nn.loss.MSELoss`

test=develop

* Move to `nn/layer/loss.py`

test=develop

* Add test

test=develop

* Fix dygraph

test=develop

* Increase numel in test

test=develop

* Add test for input with more dimensions

test=develop

* test=develop
Co-authored-by: Nwuxing03 <wuxing03@baidu.com>

Fix bug in convert_call because difference exists between python3 and python2. test=develop (#23966)

test=develop test=document_fix

restrict block num of layer_norm_grad cuda kernel to 128, test=develop

* Support LoDTEnsorArray in fetch op

test=develop

* Support LoDTensorArray in fetch

test=develop

* Support LoDTensorArray in fetch

test=develop

* Support LoDTensorArray in fetch

test=develop

* Support LoDTensorArray in fetch

test=develop

* Support LoDTensorArray in fetch

test=develop

* Support LoDTensorArray in fetch

test=develop

* Support LoDTensorArray in fetch

test=develop

* Support LoDTensorArray in fetch

test=develop

* Support LoDTensorArray in fetch

test=develop

* [Dy2Stat]Support recursive call.

* Remove Redundant decorator to pass the Py35 unittest temporarily.

* add gradient check to reduce ops

* add skip gradient check to reduce ops test=develop

* modify stack api test=develop

* add bmm op and move serval ops from fluid.layers to tensor test=develop

* add paddle.tensor
test=develop

* polish gpu where codes
test=develop

* polish test code
test=develop

[Dy2stat] Add Test and Example Code for Different Access to ProgramTranslator and Fix Related Bug (#23958)

To prepare for publishing APIs, I added tests for that we can access dy2stat through:

@fluid.dygraph.declarative
@fluid.dygraph.jit.declarative
fluid.dygraph.ProgramTranslator()
fluid.dygraph.dygraph_to_static.ProgramTranslator()
fluid.dygraph.dygraph_to_static.program_translator.ProgramTranslator()

It surprised me that we had bugs on those different usages. I have fixed them.

I also added example codes for these new APIs

This PR also pulls my current PR https://github.com/PaddlePaddle/Paddle/pull/23880, so the PR history is long. For reviewer information, you could review this PR after https://github.com/PaddlePaddle/Paddle/pull/23880 is merged

change the  exponet_tensor  to  exponent_tensor  in the pow api 

 fix compare and logical ops cond/out parameter doc bug，correct the dtype message 

1. Rename Dygraph To Static Decorators to declarative
2. dygraph_to_static_func is still using in some training tests, I cannot delete it now.
3. Add some API docs

* enhance error message test=develop

* fix CI test of 3 op test=develop

* fix bug caused by the diff of long type in py2 and py3, test=develop

* fix error message of l2_normalize, matmul, mean, etc. 
* add the test case for those ops

* Move BasicApiTransformer to a separate file. test=develop

* Fix a bug: A api in module is not a real dygraph api in dygraph_to_static. test=develop

* improve efficiency of dot op in dygraph mode

* add comments for dot op for dygraph behavior

* fix trt fc fuse test, test=develop

* fix trt_transpose_flatten_concat shape, test=develop

* add addcmul, test=develop

* OP clip, merge_lod_tensor, convert/elementwise error message enhancement. test=develop

test=develop

As the title

* add nll_loss op test=develop

* Weight quantization support channel_wise_abs_max method to achieve higher accuracy

* add paddle.std api test=develop

* update  test=develop

* fix example code format test=develop

* allow amp and recompute working together

API/OP (center_loss, fluid.one_hot, prroi_pool, roi_pool, ctc_greed_decoder) error message enhancement (#23794)

* error message enchanced, test=develop

* error message enchanced for APIs, test=develop

* error message enhanced for roi_pool, test=develop

* update added code, test=develop

* update fluid/input.py, test=develop

* update fluid/input.py ,test=develop

* fix code style, test=develop

* fix lower converage, test=develop

* error message enhanced for roi pool, test=develop

As the title.

* fix bug in reduce op test=develop

* fix bug in reduce op test=develop