Add the ReLU6, Tanhshrink, SELU, Softplus, Softshrink and Softsign for the api 2.0

[hong19860320]

PR types

Others

PR changes

APIs

Describe

新增/修改如下2.0 API:

ReLU6:

paddle.nn.ReLU6(name=None) - 新增
paddle.nn.functional.relu6(x, name=None) - 新增，去掉threshold参数（fluid.layers.relu6包含threshold参数）

Tanhshrink:

paddle.nn.Tanhshrink(name=None) - 新增
paddle.nn.functional.tanhshrink(x, name=None) - 新增，修改名称（fluid.layers.tanh_shrink）

SELU:
paddle.nn.SELU(scale=1.0507009873554804934193349852946, alpha=1.6732632423543772848170429916717, name=None) - 新增
paddle.nn.functional.selu(x, scale=1.0507009873554804934193349852946, alpha=1.6732632423543772848170429916717, name=None) - 新增，设置参数默认值（fluid.layers.selu(x, scale=None, alpha=None, name=None)）

Softplus:
paddle.nn.Softplus(beta=1, threshold=20, name=None) - 新增
paddle.nn.functional.softplus(x, beta=1, threshold=20, name=None) - 新增，增加参数beta和threshold参数，并设置默认值（fluid.layers.softplus(x, name=None)）

Softshrink:
paddle.nn.Softshrink(threshold=0.5, name=None) - 新增
paddle.nn.functional.softshrink(x, threshold=0.5, name=None) - 新增，alpha改为threshold并设置默认值（fluid.layers.softshrink(x, alpha=None, name=None)）

Softsign:
paddle.nn.Softsign(name=None) - 新增
paddle.nn.functional.softsign(x, name=None) - 新增，（fluid.layers.softsign(x, name=None)）

[paddle-bot-old]

Thanks for your contribution!
Please wait for the result of CI firstly. See Paddle CI Manual for details.

[paddle-bot-old]

✅ This PR's description meets the template requirements!
Please wait for other CI results.

[zhupengyang]

LGTM