add common dtypes as paddle's dtypes

[iclementine]

PR types

Function optimization

PR changes

APIs

Describe

add common dtypes as attribute of paddle. paddle.float32, paddle.float64 are now objects. They can be used as dtype argument in many functions.

[paddle-bot-old]

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[willthefrog]

this approach would result in similar behavior as paddle.Tensor, i.e., print(uint8) # VarType.UINT8

[iclementine]

this approach would result in similar behavior as paddle.Tensor, i.e., print(uint8) # VarType.UINT8

The repr of VarDesc.VarType has already been change by

Paddle/python/paddle/fluid/dygraph/varbase_patch_methods.py

Line 448 in cd74b20

# NOTE(zhiqiu): pybind11 will set a default __str__ method of enum class.

what I meant is the type of dtype, e.g., type(uint8) or print(uint8)

[willthefrog]

LGTM, @jzhang533 @XiaoguangHu01

[jzhang533]

lgtm

[XiaoguangHu01]

LGTM