The ELU activation is added (#8699)

* tanh function been added

* tanh function been added

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* tanh function is added

* tanh function is added

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* tanh function added

* tanh function added

* tanh function is added

* Apply suggestions from code review

* ELU activation function is added

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* elu activation is added

* ELU activation is added

* Update maths/elu_activation.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Exponential_linear_unit activation is added

* Exponential_linear_unit activation is added

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>