feat(backends): Added a primary implementation for flatten in tensorflow backend

[hmahmood24]

PR Description

Adds a primary implementation for flatten in the tensorflow backend which is compatible with tf.function. The implementation is taken from transformers.

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• Did you add a function?
• Did you add the tests?
• Did you run your tests and are your tests passing?
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[vedpatwardhan]

I haven't run the tests for this function locally (and for some reason the test isn't being run in the CI for this one), but I assume you've already tested the function, feel free to merge. Thanks @hmahmood24 😄

[hmahmood24]

I haven't run the tests for this function locally (and for some reason the test isn't being run in the CI for this one), but I assume you've already tested the function, feel free to merge. Thanks @hmahmood24 😄

Yup the tests are passing locally. Thanks I'll merge now 😄