TPU fp16 requires apex installed
When I tried to use precision=16 on TPU, pytorch-lightning is trying to find amp, which is unnecessary.
The backtrace is
GPU available: False, used: False
TPU available: True, using: 8 TPU cores
Traceback (most recent call last):
  File "bert_ner/light/fp16_debug.py", line 16, in <module>
    trainer = pl.Trainer(tpu_cores=8, precision=16)
  File "/anaconda3/envs/torch-xla-1.5/lib/python3.6/site-packages/pytorch_lightning/trainer/trainer.py", line 607, in __init__
    self.init_amp()
  File "/anaconda3/envs/torch-xla-1.5/lib/python3.6/site-packages/pytorch_lightning/trainer/auto_mix_precision.py", line 27, in init_amp
    "You set `use_amp=True` but do not have apex installed."
ModuleNotFoundError: You set `use_amp=True` but do not have apex installed.Install apex first using this guide and rerun with use_amp=True:https://github.com/NVIDIA/apex#linux his run will NOT use 16 bit precision

To Reproduce
Steps to reproduce the behavior:
build a whatever Trainer in TPU and use fp16
Code sample

import pytorch_lightning as pl

trainer = pl.Trainer(tpu_cores=8, precision=16)

Expected behavior

Should have nothing error.
Environment

PyTorch Version (e.g., 1.5.0):
OS (e.g., Linux): Linux
How you installed PyTorch (conda, pip, source): conda
Build command you used (if compiling from source):
Python version:
CUDA/cuDNN version:
GPU models and configuration:
Any other relevant information: actually I directly use pytorch-xla-1.5 docker on Google Cloud

Additional context