Fix member declartion of deepmd and deepmd.entrypoints.

[shishaochen]

Symbol DeepEval is not imported while declared in field __all__ of deepmd/__init__.py.

>>> from deepmd import *
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'deepmd' has no attribute 'DeepEval'

Module train should not be declared as public in deepmd/entrypoints/__init__.py. Actually, train_dp is the correct one.

>>> from deepmd.entrypoints import *
>>> test
<function test at 0x7f7c2bc53d08>
>>> train
<module 'deepmd.entrypoints.train' from '/usr/local/lib/python3.7/dist-packages/deepmd_kit-2.0.0b3.dev37+g4d08f7e-py3.7-linux-x86_64.egg/deepmd/entrypoints/train.py'>

[codecov-commenter]

Codecov Report

Merging #922 (2126408) into devel (ee0ed99) will increase coverage by 7.87%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            devel     #922      +/-   ##
==========================================
+ Coverage   75.42%   83.29%   +7.87%     
==========================================
  Files          85      118      +33     
  Lines        6730     9916    +3186     
==========================================
+ Hits         5076     8260    +3184     
- Misses       1654     1656       +2     

Impacted Files	Coverage Δ
deepmd/entrypoints/__init__.py	100.00% <ø> (ø)
deepmd/__init__.py	81.81% <100.00%> (ø)
source/lib/tests/test_fmt_nlist.cc	100.00% <0.00%> (ø)
source/api_cc/tests/test_deeppot_model_devi.cc	100.00% <0.00%> (ø)
source/lib/tests/test_ewald.cc	100.00% <0.00%> (ø)
source/lib/tests/test_soft_min_switch_force.cc	100.00% <0.00%> (ø)
...ource/lib/tests/test_soft_min_switch_force_grad.cc	100.00% <0.00%> (ø)
source/lib/tests/test_prod_virial_r.cc	100.00% <0.00%> (ø)
source/api_cc/tests/test_ewald.cc	100.00% <0.00%> (ø)
source/lib/tests/test_map_aparam.cc	100.00% <0.00%> (ø)
... and 25 more

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