feat: implemented sigmoid function

[justcho5]

• implement sigmoid function in ggml
• implement metal kernel
• implement cuda kernel

This is required for encodec.cpp and bark.cpp.

[justcho5]

@slaren Hey my PR is complete. Should I implement tests?

[slaren]

In this case since it is implemented as a GGML_OP_UNARY there is already a test, all you have to do to enable it is modify the supports_op function of the backends to return true for GGML_UNARY_OP_SIGMOID. Then you can run test-backend-ops to test the CUDA and Metal implementations.

[justcho5]

Thank you! Looks like the operation passed the tests on my machine.

[PABannier]

That's super helpful! Thanks @justcho5 :)
Encodec.cpp will now be able to support the Metal backend :)