Add Benchmarks to issue templates

.github/ISSUE_TEMPLATE/---new-benchmark.md

+---
+name: "\U0001F5A5 New Benchmark"
+about: You benchmark a part of this library and would like to share your results
+title: "[Benchmark]"
+labels: ''
+assignees: ''
+
+---
+
+# Benchmarking Transformers
+
+## Benchmark
+
+Which part of Transformers did you benchmark?
+
+## Set-up
+
+What did you run your benchmarks on? Please include details, such as: CPU, GPU? If using multiple GPUs, which parallelization did you use?
+
+## Results
+
+Put your results here!