New tensors created are `float32` when using `precision=64`
🐛 Bug

New tensors created in LightningModule (e.g. in training_step) when using precision=64 should have dtype=torch.float64 rather than dtype=torch.float32 etc.
Please reproduce using the BoringModel

To Reproduce
Here:
https://colab.research.google.com/drive/15qTdFpeu37B-0hUUIyn707fbpbE5BG9e?usp=sharing

Expected behavior
Both assert statements in above notebook should pass, not just the first