Add docstrings build_utils.py, enforce consistent precision answers over a single run (#1331)

Author: mikekgfb

torchchat/utils/build_utils.py

@@ -111,16 +111,30 @@ def use_et_backend() -> bool:
 ##########################################################################
 ###          set and get target precision for this model               ###
 
-precision = torch.float32
+precision = None
 
 
 def set_precision(dtype):
+    """set_precision() is a torchchat-internal API that records the dtype we're building the model for.  
+The precision is recorded for future queries by get_precision(), so that when building a model,
+or performing optimizations, we can query the type the user is building the model for. 
+This is an informational value that can be used when we want to know what type to build for (e.g., a kv cache).  
+Changing the `precision` does not change the precision of the model.
+"""
+    
     global precision
+    assert precision is None, "only set precision once to avoid inconsistent answers during different phases of model build and export"
     precision = dtype
 
 
 def get_precision():
+    """get_precision() is a torchchat-internal API that returns the dtype we're building the model for, as specified by the `--dtype` CLI option+,
+or the precision quantizer.
+"""
     global precision
+    # if (and only if) precision has not been set, update it to the default value torch.float32
+    if precision is None:
+        precision = torch.float32
     return precision