RuntimeError: "triu_tril_cuda_template" not implemented for 'BFloat16'
### System Info

transformers==4.19.2

### Who can help?

use bf16 with accelerate config
```
compute_environment: LOCAL_MACHINE
deepspeed_config: {}
distributed_type: MULTI_GPU
fsdp_config: {}
machine_rank: 0
main_process_ip: null
main_process_port: null
main_training_function: main
mixed_precision: bf16
num_machines: 1
num_processes: 8
use_cpu: false
```
model is LongformerModel, and get error

File "~/miniconda3/envs/bai/lib/python3.8/site-packages/transformers/models/longformer/modeling_longformer.py", line 788, in _mask_invalid_locations
    beginning_mask_2d = input_tensor.new_ones(affected_seq_len, affected_seq_len + 1).tril().flip(dims=[0])
RuntimeError: "triu_tril_cuda_template" not implemented for 'BFloat16'


### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

none

### Expected behavior

none
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