docs: remove string optimizers from examples

[aaron276h]

No description provided.

[neilconway]

Should we change any of these?

$ ag "optimizer=\""
docs/tutorials/native-tf-keras/tf_keras_native_dtrain.py
50:model.compile(optimizer="adam", loss="sparse_categorical_crossentropy", metrics=["accuracy"])

docs/tutorials/native-tf-keras/tf_keras_native.py
25:model.compile(optimizer="adam", loss="sparse_categorical_crossentropy", metrics=["accuracy"])
54:model.compile(optimizer="adam", loss="sparse_categorical_crossentropy", metrics=["accuracy"])

docs/tutorials/native-tf-keras/tf_keras_native_hparam_search.py
57:model.compile(optimizer="adam", loss="sparse_categorical_crossentropy", metrics=["accuracy"])

docs/tutorials/native-tf-keras/tf_keras_native_hparam_search.rst
73:    model.compile(optimizer="adam", loss="sparse_categorical_crossentropy", metrics=["accuracy"])

docs/tutorials/native-tf-keras/tf_keras_native.rst
38:    model.compile(optimizer="adam", loss="sparse_categorical_crossentropy", metrics=["accuracy"])
70:    model.compile(optimizer="adam", loss="sparse_categorical_crossentropy", metrics=["accuracy"])

docs/tutorials/native-tf-keras/tf_keras_native_dtrain.rst
66:    model.compile(optimizer="adam", loss="sparse_categorical_crossentropy", metrics=["accuracy"])

[aaron276h]

@neilconway those docs aren't part of the repo