[SYCL][DOC] Initial Draft of Extension for querying free device memory on Level Zero (#3468)

Signed-off-by: James Brodman <[email protected]>

Author: jbrodman

sycl/doc/extensions/LevelZeroBackend/LevelZeroBackend.md

@@ -139,9 +139,54 @@ Applications must make sure that the Level-Zero handles themselves aren't used s
 Practically speaking, and taking into account that SYCL runtime takes ownership of the Level-Zero handles,
 the application should not attempt further direct use of those handles.
 
+## 5 Level-Zero additional functionality
+
+### 5.1 Device Information Descriptors
+The Level Zero backend provides the following device information descriptors
+that an application can use to query information about a Level Zero device.
+Applications use these queries via the `device::get_backend_info<>()` member
+function as shown in the example below (which illustrates the `free_memory`
+query):
+
+``` C++
+sycl::queue Queue;
+auto Device = Queue.get_device();
+
+size_t freeMemory =
+  Device.get_backend_info<sycl::ext::oneapi::level_zero::info::device::free_memory>();
+```
+
+New descriptors added as part of this specification are described in the table below and in the subsequent synopsis.
+
+| Descriptor | Description |
+| ---------- | ----------- |
+| `sycl::ext::oneapi::level_zero::info::device::free_memory` | Returns the number of bytes of free memory for the device. |
+
+
+``` C++
+namespace sycl{
+namespace ext {
+namespace oneapi{
+namespace level_zero {
+namespace info {
+namespace device {
+
+struct free_memory {
+    using return_type = size_t;
+};
+
+} // namespace device;
+} // namespace info
+} // namespace level_zero
+} // namespace oneapi
+} // namespace ext
+} // namespace sycl
+```
+
 ## Revision History
 |Rev|Date|Author|Changes|
 |-------------|:------------|:------------|:------------|
 |1|2021-01-26|Sergey Maslov|Initial public working draft
 |2|2021-02-22|Sergey Maslov|Introduced explicit ownership for context
+|3|2021-04-13|James Brodman|Free Memory Query