* Add `paddle.nn.loss.MSELoss`

test=develop

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

test=develop

* Fix dygraph

test=develop

* Add test

test=develop

* Increase numel in test

test=develop

* Add test for input with different dimensions

test=develop

optimize compare and logical ops error info, add test case for this ops， add some test case for those ops

cherry-pick #23645

[Cherry-pick 2.0] Error message enhancement of conv2d_fusion and bugfix related to control nodes detecttion in fusion_group (#23985)

* Op(conv2d_fusion) error message enhancement. (#23596)
* Fix the error misjudgment when there are control nodes in graph. (#23943)

* OP error message enhancement of l2_normalize, matmul, mean, etc 

* [Cherry-pick][dy2static]Support recursive call (#23900)
  * Support recursive call.

  * Remove Redundant decorator to pass the Py35 unittest temporarily.

* [Cherry-pick]Fix bug in convert_call because difference exists between python3 and python2. (#23966 )

* allow amp and recompute working together

[Dy2stat] Add Test and Example Code for Different Access to ProgramTranslator and Fix Related Bug (#23958) (#23963)

Cherry pick of 23958

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

* Avoid logging.info be printed many times in dygraph_mode,test=develop

* Avoid logging.info be printed many times in dygraph_mode,test=develop

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

* add addcmul, test=release/2.0

* test=develop fix api doc error

test=release/2.0

Fix the support the float16 of scale op, add delete the raise case for fp16

* add approximation for gelu, test=develop

* add functional conv

* add test and doc for function convs, test=develop

* update ConvTransposeOp's InferShape and error message, test=develop

* add hsigmoid, row_conv in paddle.nn(functional and layer), test=develop

* fix hyperlinks in docstring

* correct reader device index, test=develop

* fix async executor scope var initialization, test=release/2.0

* Add variance computing layer for tensor

* Format input data types

* Remove unnecessary expand operation

* Some fixes in doc

add new tensor: max,min,t,eye,log1p; test=develop

* api compild program error polish, test=develop

* fix coverage problem, test=develop

* fix details & add unittests, test=develop

* add test for coverage, test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* aAdd new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add the support of bool list for assign value, test=develop

* Fix the assign op test case for bool dtype, test=develop

* Revert to usage of 'assign' in test_transformer. test=develop
Co-authored-by: Nwawltor <fangzeyang0904@hotmail.com>

* Add the support of bool list for assign value, test=develop
* Fix the assign op test case for bool dtype, test=develop

* Add new norm api, support frobenius norm and p-order vector norm. test==develop
* combine test files, add more attr checks. test=develop

* add unit accessor in fleet, support DownpourUnitAccessor
* test=develop

* add cuda kernel for seed, test=develop

* polish error message, test=develop

* enhance error msg. test=develop

* print invalid argment

* update comment, test=develop

* fix enforce not meet specification

* fix enforce not meet specification, test=develop

1. update error info of the ops (abs, acos, asin, atan, ceil, cos, exp, floor, log, pow, reciprocal, round, rsqrt, sin, sqrt, square, tanh)
2. add the unittests of the above refered ops (test error info)

API(has_inf, has_nan, cumsum, fsp_matrix, increment) error message enhancement, test=develop (#23769)

* API(argsort, argmax, argmin, cast, diag) add error message，and the same time add the test cases for those apis.

This PR enhances error messages of several API/OPs:

ParallelExecutor (python && C++)
Executor (python && C++)
StaticRNN (python)
IfElse (python)
cond (python)
split_lod_tensor (python && C++)

Those ops include，scale， sum， sums，unique_with_counts，unique，
wherre， add error message and test case