Minor fixes (#3059)

* Minor fixes to the flight recorder tutorial

Summary:
1. Fix a sentence saying that there 3 required parameters to enable this feature. Clarify default values.
2. Add a link for the relevant code that user needs to modify to write out Flight Recorder dumps to a location of their choosing.
3. Clarify a sentence by adding the switch that customers need to use to filter their data by ranks.

Test Plan:
Looked at renderings locally and also in the tests.

Author: c-p-i-o

prototype_source/flight_recorder_tutorial.rst

@@ -46,15 +46,15 @@ Flight Recorder consists of two core parts:
 
 Enabling Flight Recorder
 ------------------------
-There are two required environment variables to get the initial version of Flight Recorder working.
+There are three required environment variables to get the initial version of Flight Recorder working.
 
-- ``TORCH_NCCL_DEBUG_INFO_TEMP_FILE``: Setting the path where the flight recorder will be dumped with file prefix. One file per
-  rank. The default value is ``/tmp/nccl_trace_rank_``.
 - ``TORCH_NCCL_TRACE_BUFFER_SIZE = (0, N)``: Setting ``N`` to a positive number enables collection.
   ``N`` represents the number of entries that will be kept internally in a circular buffer.
-  We recommended to set this value at *2000*.
+  We recommended to set this value at *2000*. The default value is ``2000``.
 - ``TORCH_NCCL_DUMP_ON_TIMEOUT = (true, false)``: Setting this to ``true`` will write out diagnostic files to disk on job timeout.
-  If enabled, there will be one file per rank output in the job's running directory.
+  If enabled, there will be one file per rank output in the job's running directory. The default value is ``false``.
+- ``TORCH_NCCL_DEBUG_INFO_TEMP_FILE``: Setting the path where the flight recorder will be dumped with file prefix. One file per
+  rank. The default value is ``/tmp/nccl_trace_rank_``.
 
 **Optional settings:**
 
@@ -74,7 +74,8 @@ Additional Settings
      ``fast`` is a new experimental mode that is shown to be much faster than the traditional ``addr2line``.
      Use this setting in conjunction with ``TORCH_NCCL_TRACE_CPP_STACK`` to collect C++ traces in the Flight Recorder data.
 - If you prefer not to have the flight recorder data dumped into the local disk but rather onto your own storage, you can define your own writer class.
-  This class should inherit from class ``::c10d::DebugInfoWriter`` and then register the new writer using ``::c10d::DebugInfoWriter::registerWriter``
+  This class should inherit from class ``::c10d::DebugInfoWriter`` `(code) <https://github.com/pytorch/pytorch/blob/release/2.5/torch/csrc/distributed/c10d/NCCLUtils.hpp#L237>`__
+  and then register the new writer using ``::c10d::DebugInfoWriter::registerWriter`` `(code) <https://github.com/pytorch/pytorch/blob/release/2.5/torch/csrc/distributed/c10d/NCCLUtils.hpp#L242>`__
   before we initiate PyTorch distributed.
 
 Retrieving Flight Recorder Data via an API
@@ -189,7 +190,7 @@ command directly:
 Currently, we support two modes for the analyzer script. The first mode allows the script to apply some heuristics to the parsed flight
 recorder dumps to generate a report identifying potential culprits for the timeout. The second mode is simply outputs the raw dumps.
 By default, the script prints flight recoder dumps for all ranks and all ``ProcessGroups``(PGs). This can be narrowed down to certain
-ranks and PGs. An example command is:
+ranks and PGs using the *--selected-ranks* argument. An example command is:
 
 Caveat: tabulate module is needed, so you might need pip install it first.