Various fixes to mixed precision doc.
Various fixes to mixed precision doc.

Changes include:
1. Explicitly state TF 2.1 is required.
2. Do not use deprecated tf.test.is_gpu_available call, which outputed warning in guide.
3. Fix incorrect symbol name for LossScaleOptimizer
4. In "Ensuring GPU Tensor Cores are used" section, do not use monospaced code font for method names. It was very difficult to tell what text is bolded in the guide when it is in code font. See https://www.tensorflow.org/guide/keras/mixed_precision#ensuring_gpu_tensor_cores_are_used.

PiperOrigin-RevId: 286088347

tensorflow/docs,tensorflow/docs,tensorflow/docs