Add DRAM_SSD backend type to BackendType enum

Summary:
Add `DRAM_SSD` as a new backend type to the fbgemm `BackendType` enum in
`fbgemm_gpu/tbe/ssd/ssd_config.py`. `DRAM_SSD` represents a composite DRAM + SSD
KV backend for KVZCH virtual tables and is required by the
`DRAM_SSD_VIRTUAL_TABLE` compute kernel, which references `BackendType.DRAM_SSD`.

- Add `DRAM_SSD = 3` to `BackendType`.
- Add `"dram_ssd"` mapping to `BackendType.from_str`.

Differential Revision: D106733255

Author: Lizhe Ji

fbgemm_gpu/fbgemm_gpu/tbe/ssd/ssd_config.py

@@ -312,13 +312,15 @@ class BackendType(enum.IntEnum):
     SSD = 0
     DRAM = 1
     PS = 2
+    DRAM_SSD = 3
 
     @classmethod
     def from_str(cls, key: str) -> "BackendType":
         lookup = {
             "ssd": BackendType.SSD,
             "dram": BackendType.DRAM,
             "ps": BackendType.PS,
+            "dram_ssd": BackendType.DRAM_SSD,
         }
         if key in lookup:
             return lookup[key]