Add support for instance group of type 'MODEL' (#107)

* Add support for instance group of type 'MODEL'

* Format

* Handle multi GPU cases when recording timestamps

* Address comment

* Use callback function to record timestamp for 'MODEL' kind

* Add missing #ifdef

* Update comment and if condition for input tensor memory alloc_perference

* Fix for cuda stream. Use separate cuda callback to capture timestamp

* Add comment to mention the possible timestamp issue

* For 'KIND_MODEL', use cuda events for compute_input_duration and use callback for compute_infer_duration

* Move the cudaLaunchHostFunc from RecordBackendTimestamp function

* Fix up naming

* Fix up

* Fix up atomic initialization

* Capture the timestamp after synchronization

Author: krishung5